The barrister who investigated claims of sexual misconduct by Lib Dem MP Mike Hancock towards a female constituent found her evidence "credible", according to his report.
The report, by Nigel Pascoe QC, has until now not been officially published, but copies have been leaked.
In it, he says the woman's account deserves "compassion and respect".
A police investigation into the claims - which Mr Hancock denies - found no further action should be taken.
Mr Pascoe was appointed by Portsmouth City Council in Mr Hancock's Portsmouth South constituency to produce a report after a complaint by a female - known as Annie, which is not her real name - that the MP had abused his position as an elected representative with sexual advances between November 2009 and July 2010.
In the report, Mr Pascoe says: "I do not find the actions of Mr Hancock were motivated solely by sexual desire for [Annie], who at all times was his constituent. It follows that the true picture of the behaviour of Mr Hancock towards [Annie] is more complex and nuanced than has previously been represented in the press.
"That said, I consider that the prima facie evidence of his unwelcome sexual approaches remains unquestionably a very serious matter in the light of the position which he holds. No-one in public life should allow themselves to act in such an irresponsible and damaging way."
Sections of the 49-page report have been redacted. However, Mr Pascoe does state: "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 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.
On Tuesday, the council voted not to release a copy of the QC's report, but it has since been leaked.
The report outlines the chronology and detail of the allegations, including what are said to be text messages exchanged between Mr Hancock and the constituent, who cannot be named for legal reasons.
On Mr Hancock's website, a spokesman for the MP wrote: "These matters have twice been looked at by the police, first in 2010, and in conjunction with the CPS, they have found that there is no case for Mr Hancock to answer and have taken no further action.
"Portsmouth standards sub-committee has yet to meet to hear evidence including from Mike Hancock as it decided to postpone that hearing until after the associated civil court case."
The spokesman added: "Unfortunately due to the ongoing civil case, Mike received legal advice that he should not give evidence to the council's investigator. He 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."
Michael Lawther, Portsmouth City Council's city solicitor, said: "A code-of-conduct complaint against Cllr Hancock is going through the council's internal process. The process is not yet complete, and the allegations in the report remain allegations.
"The relevant sub-committee has reviewed the Pascoe report and has decided that the matter should go to a hearing at which Cllr Hancock and/or his legal advisers will have the opportunity to respond to the report, and question witnesses and Mr Pascoe."
In a statement, Mr Pascoe said the release of a redacted version of his report had been "unsatisfactory in a number of ways".
He said it did not carry the "full reasoning for my conclusions" and did "not set out the position of Mr Hancock as I perceived it to be, including a number of assumptions and comments that I set out clearly in his favour".
Mr Pascoe added that "some of my conclusions have been omitted, although I can understand why that might have been done at this moment in time. I should add that there are other redactions of a minor character which I also understand."
He said it "would be better for my full report to be published sooner rather than later".