Pressure continues to mount on Nick Clegg, as Liberal Democrat president Tim Farron admitted the party "screwed up" its response to allegations of improper behaviour by former chief executive Lord Rennard.

After initial denials that he was aware of complaints made by a number of women about Lord Rennard, Mr Clegg confirmed his office had heard "indirect and non-specific concerns" as far back as 2008, and had taken action at the time. In a radio interview, he said "no very specific allegations" were put to him until a Channel 4 News investigation 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.

Meanwhile, David Cameron's official spokesman declined to comment on the Rennard affair, but said the Prime Minister regarded harassment of any kind "unacceptable".

Lord Rennard has strenuously denied "any suggestion of improper touching" of women who he came into contact with in his role as chief executive.

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.

Mr Clegg said that his former chief of staff Danny Alexander - now chief secretary to the Treasury - confronted Lord Rennard after concerns were raised anonymously in 2008 and warned him that any such behaviour was "wholly unacceptable". The peer firmly denied any wrong-doing.

Speaking to BBC Radio Solent days ahead of Thursday's crunch Eastleigh by-election, Mr Clegg said: "I totally understand people have got lots and lots of questions but I hope I have given a full, frank, honest account. I have got nothing to hide, the party has nothing to hide."

Mr Farron acknowledged that the party had "screwed this up" and said a "completely full and open inquiry into how we got this wrong" was now under way.

"There are individuals out there who we had a duty of care towards and we did not fulfil that duty of care," the Lib Dem president told BBC Radio 4's Today programme. "That is something that we have to learn from, apologise for and make sure it never happens again."