The Liberal Democrats have suspended a party member and referred reported rape allegations to the police.
The allegations emerged in a series of tweets over the weekend, which claimed an alleged victim's complaint had been "hushed up" by party HQ.
The party said a member was suspended "pending the outcome" of a police probe but gave no further details.
It is the first allegation involving the Lib Dems in the misconduct scandal currently engulfing Westminster.
Lib Dem leader Sir Vince Cable earlier took part in cross-party talks which resulted in the announcement of a new grievance procedure for staff at Westminster, to be introduced in the New Year.
Lib Dem volunteers have set up a website, LibDemsToo, to catalogue "accounts of sexism within the UK Liberal Democrats and useful links for taking action".
In a statement, a Lib Dem spokesman said: "If an allegation is made against a party member, the disciplinary procedure is triggered immediately with an investigator talking to witnesses.
"If someone has an allegation of a criminal nature against a member of the party, we would urge them to go to the police.
"If they were unable to do so, we would refer it to the police on their behalf. Our party has a clear and robust complaints procedure set out on our website.
"We take any such complaints extremely seriously.
"If anyone has any complaints, we would urge them to come forward as set out on our website, and our pastoral care officer will make prompt contact."