Nick Clegg has insisted he took immediate action after learning of "appalling" sexual allegations against backbench MP Mike Hancock.
The Liberal Democrat leader defended his handling of the situation amid claims that nothing was done for three years after the party was notified of the complaints.
The Portsmouth South MP was suspended from the party this week after the leaking of a QC's report into allegations - which he denies - of inappropriate behaviour towards a constituent with mental health issues.
Mr Hancock had already resigned the parliamentary party whip last year, but was still sitting as a Lib Dem local councillor.
Mr Clegg, who is also facing criticism over the handling of separate sexual harassment allegations against Lord Rennard, said he had acted last year as soon as he heard that a civil case was being brought against his MP.
"When I was given the specific allegations at the beginning of last year I immediately asked our chief whip to investigate," he said. "As a result Mike Hancock ceased to be a Lib Dem MP.
"When those allegations were then supported by the QC's report that has come to light this week again we acted immediately and Mike Hancock has been suspended from the Lib Dems altogether."
The Deputy Prime Minister added: "The first time I was given specific allegations was when there was a notification of legal proceedings, which I was informed of at the beginning of last year.
"You can only react to things that are put to you.
"That is when I acted. I acted immediately because I was as appalled as anyone by the seriousness of these allegations."
After a Hampshire police investigation concluded there was no case to answer, Portsmouth City Council launched an inquiry into whether Mr Hancock had breached the council's code of conduct, but that has now been put on hold pending the outcome of the civil court action.
The report, carried out by Nigel Pascoe QC for the council, found "compelling" evidence of "serious and unwelcome sexual behaviour" towards a vulnerable woman constituent. She suffered a mental health disorder and had been sexually abused as a child.
Her solicitor, Harriet Wistrich, said it had taken the Lib Dem leadership three years to take action after the complaints about his conduct first came to light.
"There seems to be a pattern here of Lib Dems not dealing with the matter," Ms Wistrich told Sky News.
"My client's wish is for him to be held properly to account for his misconduct. We are exploring all avenues. It has taken us three years to reach this stage because every time she has gone down one regulatory procedure or another, there's been no investigation or no decision made.
"I think that the Lib Dems have very belatedly suspended him from party membership. The council have been appalling in not taking action earlier. He still is a councillor, they could proceed now with the allegations that he has breached their council code of conduct."
Local Lib Dems in Portsmouth have continued to voice support for Mr Hancock, accusing the party leadership of suspending him without due process.
"We believe in proper and due process and that as there is an ongoing court case, it is the court that should rule first, having considered all the evidence, not the media or internal panels," said Simon Dodd, chairman of the Portsmouth Lib Dems.
"Mike Hancock has loyally represented the people of Portsmouth for over 40 years and he will continue to serve as an excellent constituency MP and as a councillor."
Mr Hancock's son has been charged with assault causing actual bodily harm following an incident involving a photographer, Hampshire police said.
Dean Hancock, 37, was released on bail and will appear at Fareham Magistrates' Court on February 12.
Following a private meeting this evening, Portsmouth City Council's Liberal Democrat group said Mr Hancock would retain his seat on the council's cabinet.
"This was decided because councillor Hancock is best placed to fulfil the role of cabinet member for planning, regeneration and economic development," the group said in a statement.
It went on: "Councillor Mike Hancock no longer remains a member of the Portsmouth City Council Liberal Democrat Group because, having had his membership suspended by the party at national level, it is not possible for him to remain a member of the city council group until the suspension is lifted.
"The group acknowledges that the suspension has been imposed before the issue to which suspension relates has been properly and fairly determined through due process in the courts.
"The group decided that the Independent councillor Mike Hancock should retain his seat on the cabinet of the City Council."