Energy Secretary Chris Huhne has been questioned for a second time by detectives over claims that he made his ex-wife take speeding points for him.
Vicky Pryce was also interviewed by police in Essex on Wednesday regarding the allegation, sources said.
Essex Police said they are preparing a file to be sent to the Crown Prosecution Service.
The pair were quizzed after officers obtained a court order to take possession of a tape recording in which the ex-couple apparently discuss the case.
The Sunday Times reported last month that it had been ordered by a judge at Chelmsford Crown Court to hand over the tape to Essex Police. The paper said it was considering an appeal.
The recording, disclosed by the paper in May, features a conversation between Mr Huhne, the Liberal Democrat MP, and Ms Pryce.
They were first interviewed by detectives in May over the claims that he persuaded her to accept a penalty on his behalf in March 2003 so that he could escape a driving ban.
Images of Ms Pryce's driving licence have emerged indicating that she received points for an offence in the Essex area on the date in question.
However, she is believed to have told police that she was actually at an event in London and Mr Huhne was behind the wheel.
Mr Huhne, who left Ms Pryce for another woman last year, denies the allegations. Both Prime Minister David Cameron and his Lib Dem deputy Nick Clegg have insisted there is no reason he should resign.