Vince Cable said there is "a lot of frustration" among Liberal Democrats that former party chief executive Lord Rennard cannot be expelled over his behaviour towards women.
The Business Secretary spoke out as Nick Clegg came under renewed pressure from women who claim they were sexually harrassed to take firm action against the peer.
Disciplinary proceedings against Lord Rennard were dropped after a QC concluded there was a less than 50% chance the allegations could be proved beyond reasonable doubt,.
There was however broadly credible evidence of "behaviour which violated the personal space and autonomy of the complainants", Alistair Webster's inquiry concluded.
The end of the disciplinary process, which followed a decision by the Metropolitan Police not to press criminal charges, was met with dismay by some of the complainants.
Party leader Mr Clegg has insisted the former elections supremo will play no part in the 2015 campaign.
But the peer has rejoined the Lib Dem group in the House of Lords and will return to his duties as an elected member of a key policy committee.
He is also declining to apologise to the women involved - one of the recommendations of the review - despite Mr Clegg urging him to "do the right thing".
Asked if Lord Rennard should be expelled from the party, Mr Cable said: "There is a lot of frustration after this very strong report that the party's rules do not permit that action."
But he declined to say if he would like to see that happen.
Mr Clegg and Lib Dem President Tim Farron were in close discussions "seeing how we can proceed and whether our rules need revisiting", he told BBC2's Newsnight.
"They are the key decision makers within the party, they have to operate within the rules. I don't want to add an independent view to that."
He spoke after another of the women concerned - Susan Gaszczak - waived her right to anonymity to join the chorus of complaints at the failure to take stronger action.
A member of the party's Federal Conference Committee, she said she would find it "extremely difficult" to work alongside Lord Rennard and could quit the party.
Ms Gaszczak told Channel 4 News that Mr Clegg had called her and apologised "as he did to all the women".
"He reassured me that he wants to change the rules so that this does not happen again - but he needs to be firmer," she said.
"We should not be calling for the women's resignations, we should be calling for Rennard's resignation and that should be the membership as well as the MPs."
Claiming that the rules are "weak", she said that someone at the top of the party needs to "man up" and help ensure this sort of situation cannot happen again.
"He owes me an apology and that is the minimum that he owes me," she said of Lord Rennard.
Mr Clegg says he must follow "due process".
Lord Rennard's legal representative - fellow Lib Dem Lord Carlile - said Mr Webster had concluded that the allegations could not have been proven on either the criminal standard of "beyond reasonable doubt" or the lower civil test of proof, and Lord Rennard had always denied any wrongdoing.
"There's no reason why he should (apologise) because he has denied these allegations which have not been tried," he said.
He said neither he nor Lord Rennard had seen the Webster report in full, which was a "terrible example of secret justice".