Lawyers have warned that deals could be cancelled and banks could leave the City after the UK voted by 52% to 48% to leave the European Union.

The pound dropped to its lowest level since 1985 as it became clear that Britain had voted Out, having already collapsed a historic 13% against the Japanese Yen.

Shortly after the London Stock Exchange opened the FTSE 100 fell 8%, though it later recovered slightly to down around 2.5%. The governor of the Bank of England Mark Carney attempted to calm markets saying the bank was "prepared" for Brexit and he "will not hesitate to take additional measures" as markets adjust.

The political fallout from the vote has also started with Prime Minister David Cameron pledging to resign by October this year.

Leading City law firms will have been advising clients as events unfurled throughout the night. Legal Week reported earlier in the week how a host of firms had put in place emergency hotlines to respond to clients day or night.

Clifford Chance senior partner Malcom Sweeting said: "We believe this outcome has serious implications for the City and many of our clients' businesses with exposure to the UK and the EU. We are working alongside our clients to help them as they anticipate, plan for, and manage the challenges the coming political and trade negotiations will bring."

Guy Lougher, Pinsent Masons EU and competition law head and head of the Brexit advisory team at the firm, warned some deals could be cancelled in the wake of the Brexit vote and said the firm was already advising clients on the implications for "business-critical contracts". "The sooner amendments can be agreed or terms negotiated, the more businesses can increase their sense of certainty," he said.

He added: "Deals which have been on ice or contain Brexit clauses may be renegotiated or even pulled and raising new finance in such an uncertain environment will be more challenging. It was surprising, looking at the research we undertook, how few businesses entered into discussions with investors and funders ahead of time."

Nick Elwell-Sutton, employment law partner at Clyde & Co, said to brace for job losses in the financial services sector. He said: "Unless the financial services passporting rules are resolved in the UK's favour, then many large financial services businesses are likely to relocate to within the EU meaning large scale redundancies would be highly probable."

Allen & Overy senior partner Wim Dejonghe emphasised that nothing in the legal landscape had changed immediately as a result of the pro-Brexit result.

"Long term it depends what the outcome of the potential detail will be for the economy. It's an extended period of uncertainty and that's rarely good for business," he said.

Dejonghe, on his way to a call with around 1,000 clients, said today was "a busy day" for the firm but not unprecedented.

Simmons & Simmons corporate head Mark Curtis, who is leading the firm's 20-partner strong Brexit taskforce, said: "This will be a source of great concern for many of our clients as we face an extended period of uncertainty."

We feel we are well prepared, without sounding trite, for the challenges and opportunities this will engender

Curtis said the firm's Brexit hotline is open and its Brexit Microsite has been updated early this morning with "further sector focused support and guidance". He added: "We have further client briefings planned and these are just the first steps in a long term plan as we look to support our clients in the hours, days, weeks and months of uncertainty to come."

Lee Ranson, Eversheds managing partner, said the firm was giving a "calm and measured response" to the result of the referendum to clients.

He said: "We circulated a briefing to major clients on the impact this morning – nobody knows the timeline for this – which twists and turns it will take – we feel we are well prepared, without sounding trite, for the challenges and opportunities this will engender."

When asked what the result meant for the firm's international offices he said Eversheds was "committed to a global strategy".

Susan Bright, Hogan Lovells regional managing partner UK and Africa, said the firm had already sent briefings to clients this morning and "clearly the markets are in turmoil".

We have a large number of questions coming into our corporate partners in particular

She said: "People are waking up to this and are starting to think it through, we are starting to get questions through from clients. We have a large number coming into our corporate partners in particular. Other inquiries have been around process, in terms of Section 50 and when might the UK government actually serve notice of their intention to leave. All of this is shrouded in uncertainty."

"I think it's fair to say a lot of our US clients have been intellectually interested into it up until now, but reading the odds as we all did, they didn't really expect it to happen. Now they are very invested."

Edward Braham, senior partner at Freshfields said some clients were "focused on dealing with significant uncertainty" while "others are seeing opportunities". He added: "Quality and an ability to adapt quickly to changing circumstances are at a premium at these times, and our lawyers across the world are already rising to the challenge."

Sean Connolly, London senior partner at Mayer Brown said the UK was "now in new territory". However he noted: "Whilst we have already seen an impact on the markets and can expect other unpredicted economic and business effects nothing will change until a formal notification to leave the EU is lodged with the European Council."

Asia instability In Asia partners have also been quick to react to the result as markets tumbled. Hong Kong's Hang Seng was down 4.7%, as shares in HSBC plummeted 10.8%.

Addleshaw Goddard Asia head, Nigel Francis, said: "The expectation was that ultimately it would end up as a decision to remain."

So far as the opportunity to steer Asian capital inflows to the UK is concerned, it may well be a positive

"However, [with a decision to leave], as far as the domestic practice in Hong Kong is concerned I don't think it will have a significant impact. Indeed, so far as the opportunity to steer Asian capital inflows to the UK is concerned, it may well be a positive if, as apparently is expected, asset prices and sterling fall in the short term."

K&L Gates' regional head David Tang warned that Britain's position as an "access point" for Asian businesses into the EU was likely to end.

"Those benefits may no longer be available and operations will need to be domiciled in another EU member country," he concluded.

Varying outcomes The impact of the vote is likely to vary from sector to sector. Ashurst real estate partner Hugh Lumby said there "maybe a decline in investment into the UK real estate market" as the UK negotiates its exit from the EU over the next few years.

Ashurst oil and gas partner Michael Burns said: "The most immediate concern [for the oil and gas market] is that Brexit's potential economic impact may exacerbate the already challenging environment created by lower oil prices. This, combined with the possibility of a further Scottish independence referendum, has created even more uncertainty in an existing pool of uncertainty for the industry."

King & Spalding employment partner Jules Quinn advised companies to undertake audits into their staff to assess implications post-vote. She said: "Companies should be assessing which employees will potentially be required to apply for a work permit if there is a repeal of the current EU immigration rules."

Further reaction from the market

Michael Chissick, managing partner Fieldfisher "While the UK has voted to leave the EU, Fieldfisher has not. We are a European law firm and that won't change."

Nick Thomas, senior partner at Kennedys "We are already seeing the pound against the Euro is going back up, the pound against the dollar is going back up, the pound against the Euro is higher than it was in February, the FTSE is at well over 6000, once the excitement and the emotion of today drops off it's going to be pretty much business as usual but with people thinking long and hard about what changes they need to make.

"To the extent to which the turmoil we have seen this morning is anyway extended then sadly that always gives rise to more work for lawyers.

"You look at the big boys' websites, they have spent a lot of money on what is going to be their offering in the light of Brexit and doubtless they spent a lot of money on the other side as well. They are all seeing opportunities for clients, who if they want to get out they have to sell stuff, if they want to move people around they have employment issues, if they want to move themselves they have to buy stuff, as always turmoil gives rise to more need for lawyers.

"I expect people will sit and watch the market and watch what happens in the negotiations with Europe as to what our deal looks like. Despite the bullish noises Europe is making we are a big market for them and I am sure there will be quite a lot of deals that don't look very different. People will be making their decisions in the light of that rather than fact the markets panicked today and are now climbing back up again."

Chantal-Aimee Doerries QC, chairman of the Bar "The long-term effect of Brexit on the legal services sector's contribution to the UK economy will depend significantly on the nature and terms of the post-Brexit relationship with the EU. Despite all the turbulence, however, I am confident that London will remain a leading centre for international dispute resolution. The reputation of barristers and our judiciary overseas, beyond the EU, is very high and I expect it will remain so in the years to come."

Bill Voge, chair and managing partner of Latham & Watkins "We will continue to monitor developments as the legal and commercial ramifications of the vote become apparent. We are confident that our strength in the UK and continental Europe positions us well to advise our clients and respond to their needs worldwide."

Alberto Saravalle, former managing partner at BonelliErede "Brexit may have an impact on pending contracts: some may be terminated on the basis of a material adverse change, access to financing may be more difficult, etc. Finally, each group operating in or from the UK will need to reconsider its corporate structure, its tax position, and to what extent it benefits from the current EU rules on free movement, to name just a few key points.

"The main problem is that many groups will not be able to wait to see how the negotiations between the UK and the EU unfold before they are forced to make difficult decisions."

Etay Katz, financial services regulatory partners at Allen & Overy "In the short term, [financial services] firms should be considering their existing business models, geographical positioning and future areas of growth in order to develop contingency plans that will allow them to continue accessing the European and/or UK markets post Brexit whatever the exit model. It is expected that the regulators will start requesting these plans."

Jeremy Cohen, Dentons chief executive officer for the UK, Middle East and Africa (UKMEA) "This is clearly a historic event that will have significant implications for the UK and international business community. Doubtless we will need to brace ourselves for some short term economic turbulence, but as the world's fifth largest economy with a financial centre that will retain many of its competitive advantages over other cities, there is no reason to doubt that Britain will adapt and prosper as we always have done.

"The City of London is already positioned as the most global financial centre in the world, facilitating huge trade and currency flows to and from North and South America, Asia, Australasia and Africa, as well as Europe."

David Fennell, chief executive of Gowling WLG "It's surreal. We hosted a webinar from ten to eleven with our clients, we had a host of really good questions on topics like the impact on free movement of labour, what would happen next, what will the negotiations look like in terms of a deal, what would happen with trade deals which model will we follow Norway or Switzerland."

"The main message is nothing changes in the short term until we trigger article 50 we will continue to be a member of the EU and the relevant legislation will still apply. We think most of our clients will pause and wait and see what happens over the next few weeks and months."

"The automotive sector is worried about this, the UK is a significant exporter of vehicles to the EU, and as the UK's leading automotive firm we have had a lot of dialogue with our clients about that."

"We have seen what has happened to banks shares this morning, one must be worried to the extent to which London will remain the pivotal financial centre it currently is and that will depend on the deal we can do with Europe on passporting."

"The UK is a resilient economy and whilst to some this will be a surprise I have no doubt the UK will continue to thrive and prosper."

Oliver Brettle, White & Case executive partner in London and member of the firm's executive committee "We are currently assessing the impact, but remain confident in the continued strength of our global network and in the focus of our strategy on the US and London, on global industries and on practices that matter to our clients, globally.

"We have been advising clients on Brexit issues for several months, and we will continue to provide our clients with support and guidance as they come to terms with the implications of the UK's decision to leave the EU both on the EU and on the UK."

Mike Goetz, Ropes & Gray London managing partner "It's quite a dislocating event in lots of ways but like most dislocations you'll have opportunities and winners and losers. Our London partners will take the lead on how this affects our business and strategy. There is a little bit of dust that needs to settle. Currency has dropped; does that mean banks will drop interest rates? This may be good for capital markets. However, the real drivers have not kicked in yet."

Lord Goldsmith QC, London co-managing partner at Debevoise & Plimpton "We can be certain that the regulatory and legal implications will be significant. The environment in which businesses operate here and in Europe is going to fundamentally change as EU and UK law is unpicked, and new patterns are established. Business leaders and their advisers across sectors should be prepared to adapt to this fast-changing environment.

"The next chapter begins today."

John Joyce, managing partner Addleshaw Goddard None of us can be sure of precisely what impact the decision will have on the economy and business activity and that will unfold in the coming days, weeks, months, years. Let's see whether we even come out of the EU or this is just the start of a much bigger negotiation.

The inevitable market volatility will not be comfortable either for the UK or for much of continental Europe we suspect but the world won't come to an end and business will go on.

There will remain plenty of economic activity and opportunities, as well as difficult challenges and we hope to play a leading role at the heart of the business community finding solutions and guiding people through the uncertain conditions.

We've been fielding calls from clients for a while already across our sectors and markets on a range of issues and we've brought together briefings, videos and legal opinion on a dedicated page in our website to help guide clients on their response.

Clive Thorne, IP partner at Wedlake Bell and Brexit campaigner "Many law firms have Brussels offices for the purpose of lobbying the EU Commission or to practice EU competition law. It is probable that the demand for representation in Brussels will diminish considerably with such work being undertaken in London.

"What is unlikely to change is the network of offices maintained in other European cities such as Frankfurt and Paris which primarily exist to provide services for clients who wish to do business in Germany or France.

"It may well be that there will be increased demand for UK firms to establish representation in other jurisdictions outside Europe to service trade within the UK.

"This is already happening; long established offices in Hong Kong, Singapore and Tokyo with more recent offices in China and Africa. This will inevitably continue. US and Japanese companies have recognised London as the major centre in which to litigate and arbitrate commercial disputes especially in multi-jurisdictional disputes.

"Given the choice London has already been the favoured jurisdiction over other European jurisdictions and this is likely to continue. There will be no diminution in litigation and arbitration practice."