John Moscow, the New York assistant district attorney who was lead prosecutor in the BCCI banking scandal, will launch the attack at a major money- laundering conference in Cambridge this week. He will tell delegates that a new "cottage industry has developed in London - among certain law firms: laundering money for clients".
His comments come as police investigate whether the $10bn (pounds 6.2bn) that is thought to be involved in the Russian money-laundering scandal can be traced to London lawyers.
An investigation has also been launched by the Bank of England into a case where $50m passed through a London law firm's client account at the Royal Bank of Scotland. The firm was investigated by police, but no charges were brought.
A National Crime Intelligence Service report said the source of the money was Semion Mogilevich, the Ukrainian at the centre of the $10bn investigation.
Speaking from New York, Mr Moscow said: "The phrase cottage industry was used by the New York police to describe attorneys and accountants setting up offshore shell companies to conceal companies." The problem has now been exported to London, he said.
But Calesh Bahl, the Law Society's vice-president, said: "If the assistant DA of New York has any evidence we ask him to bring it forward so we can take action."
Mr Moscow's comments come after the NCIS slammed the legal profession following revelations that out of 14,000 suspicious transactions reported to it, only 266 came from lawyers. The NCIS described the figure as "disappointing".
The Serious Fraud Office reports that the amount of alleged fraud last year was pounds 1.4bn - pounds 200m up on last year's figure. Hundreds of billions of pounds pass through the City of London every year.
Leading City lawyers confirm that not one member of their profession has been convicted of laundering money in living memory. But police say that criminals are using them more to set up offshore vehicles to conceal their true activities.
"I accept that solicitors and lawyers are obvious targets for money launderers," said David Hughes, a partner at City law firm Dibb Lupton Allsop.
"If you've got drug money, dirty money; a way of cleaning it is putting it into property. If you want to do that, you give it to lawyers. It's a concern and we take it seriously."