An independent inquiry is being sought by councillors on Portsmouth City Council into the police investigations into Mike Hancock.
Mr Hancock admitted an "inappropriate friendship" with a constituent when he was Lib Dem MP for Portsmouth South.
Hampshire Constabulary decided not to press charges following two inquiries.
Five councillors from four parties, have written to Home Secretary Theresa May and the IPCC urging them to secure a new investigation.
The signatories include Liberal Democrat group leader Gerald Vernon-Jackson who stood for and failed to win the Portsmouth South seat for the party in May.
Senior figures in the Conservative, Labour and UKIP party groups on the council also signed the letter.
They said the investigations carried out by the Hampshire force were not thorough enough and they want the case to be given to another police force.
Mr Hancock was arrested in October 2010 after a complaint was made about his behaviour towards a vulnerable constituent who had a history of mental health problems.
In a statement, Hampshire Constabulary said: "The Crown Prosecution Service received a file of evidence from Hampshire Constabulary in December 2010 concerning these allegations of sexual assault.
"The decision from the CPS was that there was insufficient evidence for a charge."
A civil action against Mr Hancock was dropped in June 2014 with the complainant agreeing to a confidential settlement and the MP issuing an apology.
He resigned from the Liberal Democrats in September 2014 after he had the whip removed.
Mr Hancock was first elected as an SDP MP for Portsmouth South between 1984 and 1987, and as a Liberal Democrat from 1997.
He remained an MP until May when he lost the seat to the Conservatives while standing as an independent.
Mr Hancock also served as a Portsmouth councillor, which he carried out alongside his MP duties, until he lost his Fratton seat in the local elections last year.
The BBC has made efforts to contact Mr Hancock, but he has yet to comment.
"They will go on trying to make mischief as much as they can, but I have no real comment about it," he told the Portsmouth News.
The five councillors who signed the letter are:
- Liberal Democrat leader Gerald Vernon-Jackson
- Labour leader John Ferrett
- Deputy Conservative leader Luke Stubbs
- Conservative councillor Steve Wemyss
- Portsmouth's UKIP group leader, Colin Galloway