The police watchdog has launched a "corruption" inquiry following allegations that an officer conspired to pervert the course of justice.
The Independent Police Complaints Commission (IPCC) is investigating a complaint about a Merseyside Police officer.
The allegations relate to a Merseyside Police investigation in 2010 which resulted in a man being arrested, charged with breaching a non-molestation order and remanded in custody.
He was subsequently acquitted at court.
The man lodged a formal complaint about the conduct of one officer involved in the investigation, the IPCC said.
Merseyside Police referred the matter to the IPCC in September 2011 and a decision has now been taken to conduct an independent investigation.
IPCC Commissioner Naseem Malik said: "Allegations of corruption have the potential to undermine confidence in an individual police force and the police service as a whole.
"It is important therefore that a thorough and fully independent investigation is conducted to establish whether there is any substance to the allegations that have been made."