The report was written by Nigel Pascoe QC, who was asked by the city council to investigate the allegations for code of conduct proceedings.
The woman, a female constituent and a young mother, says Mr Hancock made "unwelcome sexual approaches".
The leaked, redacted version of the report describes the account she gave as "credible".
Mr Hancock denies the allegations. Police investigated but said no further action should be taken.
He was suspended from the Liberal Democrats on Wednesday and currently sits as an independent MP.
The latest legal moves come in a week when the party has also been rocked by complaints against Lord Rennard, the party's former chief executive.
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