A Scotland Yard detective and three others are arrested as part of an investigation into claims that illegal payments were made to police officers.
The arrests, which form part of a long-running police investigation, took place following allegations raised during a Parliamentary select committee hearing.
Scotland Yard said the private investigators, who are former Met officers, were arrested on suspicion of bribery as part of a lengthy and complicated investigation by the directorate of professional standards.
The serving officer is a 45 year old detective constable. A fourth suspect is in custody after being arrested under suspicion of bribing a police officer.
They were held as officers from the force's directorate of professional standards raided offices of a private investigations firm it is understood.
The case is believed to be linked to the recent conviction of former Nigerian Delta State Governor James Ibori who was jailed for 13 years last month after admitting fraud totalling nearly £50m.