Mike Schwarz of the law firm Bindmans told MPs he had evidence of "serious illegality" relating to the Metropolitan Police's proceeds of corruption unit.
He alleged that three officers, Det Insp Gary Walters, Det Con John McDonald and Det Con Peter Clarke, were "key players" in an arrangement with a private investigation firm, RISC Management Ltd.
Mr Schwarz claimed law firms used the private eyes to find out information such as the "interviewing strategy" that would be used on suspects. His personal experience came from the case of James Ibori, the former Nigerian state governor jailed for 13 years last month for fraud, he said.
He told the home affairs select committee he had seen invoices from RISC to the law firm Speechly Bircham of payments "presumably to police officers". He said the records showed "half a dozen payments totalling about £20,000 over eight or nine months".
A Met spokesman said the force was investigating an allegation of illegal payments. RISC denied making payments to police officers and Speechly Bircham said it was not aware of any payments.
