The two private investigators, who previously served with the force, are also being held after raids at the London offices of Risc Management Ltd, it is understood.

The 45-year-old detective constable is being questioned on suspicion of providing information about Nigerian fraudster James Ibori in return for cash.

Mike Schwarz, a partner at Bindmans LLP, told the Home Affairs Select Committee on Tuesday of possible cash payments made by Risc to sources who were "presumably police officers or those close to the investigation".

Records showed roughly half a dozen payments amounting to about £20,000 over an
eight- or nine-month period, Mr Schwarz claims.

The private detectives - aged 53 and 58 - were both arrested on suspicion of
bribery as part of a "pre-planned operation that forms part of a long and complex investigation by the directorate of professional standards (DPS)", the Metropolitan Police said.

A statement from the force added: "The Metropolitan Police Service is investigating an allegation that illegal payments were made to police officers for information by a private investigation agency.

"The DPS referred the matter to the Independent Police Complaints Commission in October 2011 which agreed to supervise a DPS investigation into the allegations.

"Officers from the DPS have today executed search warrants at a business address in Conduit Street."

Risc was hired by the law firm representing Ibori, a former governor of one of Nigeria's richest oil-producing states who was brought to justice in London.

He was jailed for 13 years last month after admitting fraud totalling nearly £50m.