City partners have warned that law firms must pay "very close attention" to who they do business with to ensure they do not fall foul of the new Criminal Finances Act, which will usher in a new corporate offence of failure to prevent tax evasion later this month.

The Act, which comes into force on 30 September, will make law firms vicariously liable for criminal tax evasion by 'associated persons' – including contractors, suppliers, agents or lawyers at other firms – if they do not implement reasonable procedures to prevent the facilitation of criminal tax evasion.

Partners say the Act means law firms will have to ramp up their due diligence efforts to ensure they are not exposed when advising clients on tax issues, as they can be liable even if senior management was unaware of – or uninvolved in – questionable actions taken by business associates.

Jason Collins, head of tax, litigation and regulatory at Pinsent Masons, says that the Act is aimed at stopping the "Manuel from Fawlty Towers-style, we know nothing" response from businesses whose employees are found to have stepped outside of the line.

Collins says: "In the past, the explanations given by companies for many of these offences was that management did not know it was being facilitated, as it was caused by the actions of individual members of staff. The response to that was this new criminal offence."

If you are using a firm that you've plucked out of a directory, and they make recommendations which turn out to constitute tax evasion, you are potentially at risk

Berwin Leighton Paisner commercial dispute resolution partner Andrew Tuson adds: "If a lawyer is advising, the partnership can be criminally liable. Also, any activity that happens overseas can now put the firm at risk of being criminally liable in the UK."

This, Tuson says, "means that firms need to consider whether they should produce reasonable procedures to stop these crimes from happening".

He adds: "Firms now have until the end of the month to produce a risk assessment on how likely this crime is to occur and develop an implementation plan of controls that can be introduced to mitigate against those risks."

What exactly these "reasonable procedures" look like – and who implements them – is the "million-dollar question", according to Nick Williams, a partner at offshore law firm Ogier.

Williams says: "There is an expectation that there should be top-level involvement in this, so partners need to be engaged. While it is inevitable that you have to delegate the development of processes and new procedures to others within the firm, the guys at the top will need to be visibly and tangibly involved.

"You are likely to be looking at three things. The first is resourcing: have you got the right people to deal with this new challenge, and are you utilising the information available to assess risk? Secondly, there is going to need to be a reflection about whether there are particular areas of the firm that might be exploited by an employee or contractor to facilitate tax evasion. Thirdly, you have to look at due diligence and risk assessment going forward to avoid this."

Osborne Clarke private wealth partner Andrew Goodman adds that the new legislation will increase the risk for firms in referrals and in building relationships with foreign firms.

Goodman says: "One potential issue is referrals – if a professional services firm takes advice on behalf of a client from an adviser in another country, or incorporates third-party advice into their own and provides it to a client, you could be on the hook if that third party gives you dodgy advice.

"Alternately, it could be that you have a client with business in a country where you do not have an office. If you are using a firm that you don't know, or even someone you've plucked out of a directory, and they make recommendations which turn out to constitute tax evasion, you are again potentially at risk."

Taylor Wessing criminal and regulatory investigations partner David McCluskey says this increased exposure to risk means firms will have to "pay very close attention" to other law firms and services providers with which they have a relationship.

McCluskey comments: "Firms are going to have to carry out a risk assessment – and potentially substantial due diligence – on those companies and persons, and that could add more to their compliance costs.

"The definition of what is or isn't an associated person is wide enough that any law firm that gives tax-related transactional advice is going to be asking itself careful questions about who it refers clients to, on what basis, and what it knows about those people it refers to."