Not enough lawyers are reporting suspicions of money laundering by clients, according to the Financial Action Task Force on Money Laundering (FATF).
The OECD-backed agency, in its first report since it was set up last year, called for lawyers to be better trained to recognise suspicious transactions.
The UK imposes obligations on lawyers to report suspicious activities but, the report says, "to date they have made very few declarations of suspicion".
Patrick Moulette, head of FATF's secretariat in Paris, said: "It is one thing to impose obligations on the profession and another to implement them.
"If you want to implement obligations properly, you have to give examples of what is suspicious."
FATF has no statistical evidence on lawyers' involvement in money-laundering activities, but intelligence from FATF's 26 member countries suggests that it is a serious problem.
The problem is especially acute in countries where lawyers can hide behind professional secrecy to conceal transactions.
Moulette said: "Now that the banking sector is more regulated, we have observed a move away from banks to non-financial institutions and from the financial sector to the non-financial sector which is less regulated."