Scotland Yard is investigating "whether or not criminal activity has taken place" following allegations of inappropriate behaviour towards women by Liberal Democrat peer Lord Rennard.
The news of police involvement came as pressure continued to mount on Nick Clegg, with party president Tim Farron admitting the party "screwed up" its response to allegations of improper behaviour by the party's former chief executive.
After initial denials that he was aware of complaints made by a number of women about Lord Rennard, Mr Clegg last night confirmed his office had heard "indirect and non-specific concerns" as far back as 2008, and had taken action at the time.
On Monday night, a Scotland Yard spokesman said: "The Metropolitan Police Special Investigations Command has been approached by officials in the Liberal Democrat Party and is working with them to ascertain whether or not criminal activity has taken place."
The party announced the criminal lawyer Alistair Webster QC - a former chairman of the Liberal Democrat Lawyers Association - had been appointed to lead the formal internal investigation into allegations against Lord Rennard under the party's disciplinary rules.
The independent whistle-blowing authority Public Concern at Work will act as the main point of contact for anyone wishing to come forward with further allegations or information relating to the peer.
In a radio interview, Mr Clegg said that "no very specific allegations" were put to him until a Channel 4 News investigation broadcast last week. But the Daily Telegraph published a 2010 email exchange with Mr Clegg's chief of staff Jonny Oates, in which the paper gave details of the dates and locations of four alleged incidents between 2003 and 2007 involving the Lib Dem peer, and made clear it knew the identities of the women who had complained.
Following the C4N broadcast, an investigation has now been launched under party disciplinary rules, while a separate independently-chaired review will look into how the Lib Dems dealt with the initial complaints. Lib Dem president Tim Farron acknowledged the party had "screwed this up" and said a "completely full and open inquiry into how we got this wrong" was now under way.
Lord Rennard said he was "deeply shocked" by the allegations, which he "strongly disputes" and regards as a "total distortion" of his character.
In a statement on Friday, he said: "I absolutely deny any suggestion of improper touching, nor did I invite a woman to join me in my room. I am disappointed and angry that anonymous accusations from several years ago are once again being made public in this manner in a clear attempt to damage my reputation."