Nick Clegg has said a Liberal Democrat MP could be suspended from the parliamentary party over “very serious” allegations of sexual assault.
Mike Hancock is being sued in a High Court civil action by a "vulnerable" constituent, accusing him of sexual assault after she contacted him for help in a neighbour dispute.
Mr Hancock, who vigorously denied the claims, was arrested by police over the allegations in 2010 but no action was taken as the Crown Prosecution Service said there was insufficient evidence for a prosecution.
Deputy Prime Minister Mr Clegg said an internal party disciplinary hearing would take place at the earliest opportunity - possibly today - to establish whether Mr Hancock should have the whip withdrawn.
He said: "As leader of the party, you have got to react when allegations of this seriousness are made, you can't just sit there on your hands."
Mr Clegg told LBC 97.3: "When I heard yesterday that court papers had been served for some very serious allegations - which I have to stress he denies completely, which when the police looked at it they didn't take further action, so I'm not prejudging that case - I immediately asked our chief whip to start disciplinary proceedings including the possibility of suspending the party whip."
He added: "The next step is that a meeting is held between myself, my deputy Simon Hughes, the chief whip (Alistair Carmichael) and Mike Hancock. We are hoping to do that today at the very earliest possible opportunity."
Mr Hancock, MP for Portsmouth South, was also the subject of controversy when his parliamentary assistant, Russian Katia Zatuliveter, was accused by the Security Service of being a spy passing information to Moscow.
However, the 26-year-old, who admitted having an affair with the MP while he sat on the Defence Select Committee, was granted leave to remain in the UK by the Special Immigration Appeals Commission (Siac) in November 2011.
Mr Hancock said he was not aware of any disciplinary hearing planned for today and would not be able to attend if one was held.
He claimed the way he had been treated by the party was "all a bit odd".
"There is no meeting, I have not been told there's a meeting," he said. "I'm in Portsmouth, I can't be in two places at once."
Mr Hancock, who has had health difficulties and underwent major heart surgery last year, said: "I have medical appointments today and tomorrow, that's the biggest priority in my life at the moment."
The MP added: "I have had no contact with the party at all today. The last conversation I had was at 10pm last night when Alistair Carmichael rang with the press release that I had already heard from journalists. I think it's all a bit odd."