Three former police officers and a serving Scotland Yard detective have been questioned over the police bribery case.
As part of the investigation, all the four were arrested and released on bail while questioning will continue.
The arrests were part of a planned operation set up in October 2011, said the Metropolitan Police.
The investigation comes after allegations were made that the members of an anti-corruption unit have been paid thousands of pounds as bribe.
The serving detective was arrested on suspicion at a residential address outside London, while two of the former officials were arrested in central London and the third was detained in south London.
The 45-year-old detective was quizzed amid allegations he provided information to the Nigerian money launderer James Ibori for a payment.
Ibori was a former state governor in Nigeria who stole a millions of pounds from his homeland and has now been sentenced to prison for 13 years. Ibori recently pleaded guilty to money laundering in the UK.
According to a Guardian report, one of the former officers was identified to be Keith Hunter, a former Met detective and the chief executive of the private investigation company RISC Management.
Before his arrest, the former detective told the Guardian: "RISC management does not need to pay serving police officers for confidential information as we pride ourselves on our ability to provide positive solutions and accurate information legitimately. RISC management has a highly respected reputation for conducting professional investigations." Hunter also denied any wrongdoing.
"[His company] was proud to have a network of highly skilled professional consultants, contacts and resources. These individuals are hired precisely because of their unique skill set and expertise," the Guardian quoted Hunter as saying.
The identities of other former detectives remain unknown.
A statement released by the Met police said: "The Metropolitan Police Service is investigating an allegation that illegal payments were made to police officers for information by a private investigation agency," reported the Independent.
"The Directorate of Professional Standards (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," added the statement.
Details of the case were brought to the attention of parliament on Tuesday when Mike Schwarz, a lawyer for one of Ibori's co-accused, gave evidence to the home affairs select committee about the case.
Schwarz, from Bindmans solicitors, gave details about a set of invoices he said suggested RISC Management had paid around £20,000 to sources close to the police investigation into Ibori.
The committee is holding an inquiry into whether private investigators should be subject to statutory regulation. Schwarz, who represents Bhadresh Gohil, a London-based solicitor jailed along with Ibori for orchestrating his money-laundering scam, told MPs the case amounted to an undetected case of "apparent corruption right at the heart of Scotland Yard".