MP Mike Hancock has been suspended by the Lib Dems after a report into claims of sexual misconduct towards a female constituent was leaked.
The report, by Nigel Pascoe QC, said the woman deserved "compassion and respect" and that the account she gave was "credible".
A Lib Dem spokesman said Mr Hancock's party membership had been suspended.
A police investigation into the claims - which Mr Hancock denies - found no further action should be taken.
Mr Hancock currently sits in the Commons as an independent MP, having stood down from the Lib Dems in Parliament to contest allegations of sexual impropriety in a High Court civil action.
Mr Pascoe's report has until now not been officially published, but redacted copies have been leaked.
Mr Hancock is accused by the woman - known in media reports of the case as Annie, which is not her real name - of misconduct.
The report says: "Making full allowances for the disclosed mental history of [Annie], I have no doubt that she has provided before me compelling prima facie evidence of serious and unwelcome sexual behaviour towards her by Mr Hancock. I am of the clear view that her account is credible and merits both compassion and respect."
A Lib Dem spokesman said: "The Liberal Democrats have this afternoon, for the first time, had sight of a Portsmouth City Council report by Nigel Pascoe QC into allegations of sexual impropriety by Mike Hancock."
He added: "Given Nigel Pascoe QC's conclusions in his report, we have immediately suspended Mike Hancock's membership of the party."
Mr Pascoe was appointed by Portsmouth City Council in Mr Hancock's Portsmouth South constituency to produce a report after a complaint that the MP had abused his position as an elected representative with sexual advances between November 2009 and July 2010.
A police investigation into the allegations found last year that no further action should be taken.
Mr Hancock, who is also a city councillor, temporarily stood down from the parliamentary Liberal Democrat party in June and is currently involved in a High Court civil dispute over the sexual assault allegations.
In November, a meeting of the council's standards committee decided not to proceed with any investigation into Mr Hancock until court proceedings had concluded.
Before the suspension from the Lib Dems, a spokesman for the MP said the sub-committee had yet to meet to hear evidence from Mr Hancock, as it had decided to delay that hearing until after the High Court case was over.
He added: "[Mr Hancock] looks forward to giving his evidence when the sub-committee meets after the conclusion of the civil case.
"Mike Hancock asked to meet with the council's investigator to explain this and this was denied him despite the city council's solicitor asking the investigator to meet Mr Hancock. He therefore believes the report which he has not seen is likely to be one-sided."