LONDON Critics say it doesn't go far enough, has been watered down, may barely be enforced and its terms have - perhaps deliberately - been left vague.
But Britain's Bribery Act - which covers overseas operations of firms doing business in the UK - becomes law on July 1, and some companies claim it adds new costs and may put them at a disadvantage to less tightly regulated foreign competitors.
Officials say the law will bring Britain into line with other developed countries, end any suggestion the UK ignores such issues and help stop the corruption that campaigners say keeps some poor states from developing.
While large international firms say it will make little difference to them -- they must already comply with tighter U.S. regulations -- smaller companies are less sanguine.
The act -- initially produced by Britain's former Labor government before it lost power in May 2010 -- had been due to come into force earlier this year, but was delayed in part because of frantic corporate lobbying.
The law itself was left unchanged but crucial "guidance" attached to it was subtly altered. Transparency campaigners say that added loopholes and sent the wrong message but legal experts say it still brings with it a host of new risks.
"The Bribery Act marks the start of a new era of enforcement," says Robert Amaee, a former head of anti-corruption at Britain's Serious Fraud Office(SFO) now working for law firm Covington and Burling.
"Any company that carries on a business in the UK can now incur liability for the corrupt actions (anywhere) of rogue employees or associated third parties, even if the company's management knew nothing about the bribery."
Some companies say the act does not take into account the complexities of operating in emerging economies where corruption has long been a fact of life.
With much of the FTSE and smaller AIM and other UK stock markets made up of mining, energy and infrastructure firms, there could be a lot at stake.
The last-minute guidance change theoretically exempts firms which are listed in London but operate almost entirely overseas. But lawyers disagree over exactly who is covered.
"It is a very difficult act and we have had to take it incredibly seriously," said Peter Hambro, chairman of UK-based Russian mining company Petropavlovsk.
"It is very hard to implement this sort of thing in a country where for hundreds of years the way the administration was remunerated was by taking a percentage of the taxes they collected. Undoing that mindset is not possible."
Some officials disagree. The "Arab spring," they say, has reminded governments of the potential political costs of corruption and may prompt them to seek deals with firms they know will not make matters worse.
They also say Britain had little choice but to tighten its legislation into line with North America and Europe.
"I'm not going to say that loss of business is not going to happen," Roderick Macaulay, Bribery Act implementation manager for the Ministry of Justice, told a conference in London last month after several firms said they were already suffering.
"But in the longer term, putting in place the Bribery Act will help us and ultimately it will be an advantage."
Officials also pledged to put the full weight of local embassies behind companies to help them tackle extortion. Diplomatic complaints, visa bans for corrupt officials and even aid cuts could be threatened, they say.
In the immediate term, firms are rushing to show that they have improved their internal compliance and monitoring procedures to take the act into account.
"It's a level of bureaucracy that we can do without," said Nick Clarke, chief executive of Central Asia Metals (CAML.L), a firm listed on the London AIM index with projects in Kazakhstan and Mongolia.
"However, it is more of a paper exercise... I can't always be sitting on all my people... the best thing I can do is get the paperwork in place."
But officials caution against a "tick-box" approach. There might be some room for short-term flexibility around "facilitation payments" and hospitality, they say, but most of the time staff will know when they are crossing the line.
"Bribery is like pornography -- you know it when you see it," says Nick van Benschoten, head of the anti-corruption unit at Britain's Department of Business, Innovation and Skills.
"The SFO (Serious Fraud Office) will ask why you have such rigorous controls to protect your intellectual property... but not to stop bribery."
The act requires firms to have "adequate" provisions against bribery. As with much else, plenty is open to interpretation -- and that could prove a field day for lawyers.
"The word 'adequate' is not defined," says Eoin O'Shea, a partner at London law firm Lawrence Graham. "The meaning of 'bribery' will vary from case to case."
"The Ministry of Justice guidance is a useful document but it contains elements which I would not advise my clients to accept as gospel. The principles are sometimes vague and misleading."
But transparency campaigners say their main worry is that -- with the SFO notoriously overstretched and currently facing a merger with a new anti-crime agency -- it may barely be enforced.
A few firms may "self-report" to the authorities to avoid trials, they say, but many may be left untouched.
"We would expect there to be one or two high-profile prosecutions quite quickly but I think they will be smaller companies because that's all they have the capability to deal with," said Graham Hand, coordinator of the UK Anti-Corruption Forum and a former British diplomat.
"If you were to argue that was unfair, I'd probably agree with you."
(Additional reporting by Kirstin Ridley)