Hogan & Hartson is the latest firm to benefit from the stream of unsolicited bid activity with the firm's London arm picking up a mandate to advise on this month's bid for Canada's LionOre Mining.

The US firm is advising Russia's Norilsk Nickel on its $4.8bn (£2.42bn) unsolicited bid, which tops a previous agreed bid made by Switzerland's Xstrata in March.

The deal is being led by Hogan's London arm, with US-qualified corporate partner Todd Schafer leading. Freshfields Bruckhaus Deringer and Canada's Davies Ward Phillips & Vineberg are advising Xstrata with McCarthy Tetrault acting for LionOre.

The bid comes amid a sharp upturn in aggressive bid tactics as acquirers hunt for a limited pool of coveted assets. Though few bids are turning into outright hostile takeover attempts, advisers report a growing willingness from bidders to appeal over the heads of target companies' management to shareholders.

Such tactics have even become common from private equity acquirers, who once avoided such deals.

Hogan London corporate head Jonathan Coppin told Legal Week: "There is now so much appetite from companies as well as private equity houses, things are getting very congested. There is more chance that someone else will come in with a competing proposal after an offer has already been recommended. What has changed is that competing bidders are increasingly prepared to go over the board's head to shareholders and make an unsolicited offer."

The trend has been a boost for M&A advisers as it has sharply increased the number of bidders in play, a trend amplified by the current popularity of auction-driven disposals. Likewise, the importance of regulatory and competition issues in the majority of big-ticket bids has put legal advisers in the driving seat in many deals.

For example, this year's most high profile unsolicited deal – the bidding war for ABN Amro between Royal Bank of Scotland and Barclays – has generated roles for a raft of firms. Linklaters, De Brauw Blackstone Westbroek, Slaughter and May, Clifford Chance and NautaDutilh are just some of the firms already involved.

Meanwhile, Weil Gotshal & Manges' London arm won a coveted instruction for Terra Firma when the private equity house entered the fray for Alliance Boots.

The FTSE 100 company was ultimately sold to the original bidder, Kohlberg Kravis Roberts & Co, after the US private equity house substantially raised its original offer to secure the UK's largest ever take-private.

Other examples of unsolicited bid activity this year include Spanish construction company's Sacyr Vallehermoso's A6.6bn (£4.5bn) hostile bid for French infrastructure group Eiffage in April. This saw Bredin Brat advise Eiffage with Jeantet Associes advising the Spanish company.

Similarly Homburger and Skadden Arps Slate Meagher & Flom advised reinsurance company SCOR on its hostile approach to French rival Converium Holding for A1.758bn (£1.19bn). Skadden, Simpson Thacher & Bartlett and Fried Frank Harris Shriver & Jacobson are currently advising on News Corporation's $5bn (£2.5bn) bid for Dow Jones in the US, which was rejected last week despite an aggressive premium.

The increase in competitive and hostile bids is also good news for firms' revenues. Fees on hostile bids can be as much as five times as high as on a recommended offer, while a competitive bid situation often also generate a substantial uplift for both target and bidder advisers.

Lovells corporate partner Nigel Read commented: "The bids are often very large and high profile and, providing you are not doing the bid on a contingent basis, you make more for this type of work."