A Scotland Yard anti-corruption detective and three ex-officers with the force are being investigated in connection with a suspected bribery scam.
The 45-year-old detective constable was questioned on Wednesday on suspicion of providing information about Nigerian fraudster James Ibori in return for cash.
Two private investigators, aged 53 and 58, and another man were held in connection with providing the money after raids at the London offices of Risc Management Ltd, it is understood. All four were released on bail last night pending further inquiries.
The Metropolitan Police confirmed the firm previously worked with the force.
Mike Schwarz, a partner at Bindmans LLP, told the Home Affairs Select Committee on Tuesday of possible cash payments made by investigation firm Risc to sources of theirs 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 claimed.
Scotland Yard said the fourth man was arrested at a south London police station by appointment. "He was arrested on suspicion of bribery of a police officer," a spokesman said. "Officers have previously attended a residential address outside of London and a 45-year-old man was arrested on suspicion of receiving payments for information. He is a serving detective constable in the MPS (Metropolitan Police Service)."
The two private detectives were arrested at Risc's business address as part of a "pre-planned operation that forms part of a long and complex investigation by the directorate of professional standards (DPS)".
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 £50 million.
Mr Schwarz, whose firm represented one of Ibori's co-accused, told MPs: "What I have seen in that case is serious illegality on behalf of private investigators Risc Management Ltd ... which involves apparent corruption right at the heart of New Scotland Yard. The core as I see it is the operation of the police and their connection with the security firm involved, the police being the proceeds of corruption unit within New Scotland Yard. And the problem there is the key culprits appear to be the key players, who are the senior investigating police officer ... and two of the key investigators."