Sir Menzies Campbell declared yesterday that he had been unable to escape the “cloying blanket” of questions about his age and leadership and insisted that his decision to stand down was his own and not forced upon him.

As Nick Clegg and Chris Huhne prepared to announce their campaigns to succeed him as Liberal Democrat leader, Sir Menzies invited television crews to his Edinburgh home to speak of his frustration and irritation at having to leave the stage.

Although he maintained that he had not been pushed, The Times can disclose that Sir Menzies decided to leave his post after being told by a group of close friends and confidants that the game was up.

The message was conveyed on Monday morning by his old ally, the former MP Lord Kirkwood of Kirkhope, who, at Sir Menzies’s suggestion, had canvassed opinion over the weekend. Among those whom Lord Kirkwood consulted was the former leader Lord Ashdown of Norton-sub-Hamdon, who confirmed that his advice would have been to tell Sir Menzies to go at a time of his own choosing but that it would be tough to continue until 2009.

Friends said that Sir Menzies would have regarded Lord Kirkwood’s conclusions that it was better to leave with dignity rather than suffer a protracted execution as utterly fair and impartial because he was a loyalist and friend.

However, last night Sir Menzies suggested that supporters of Mr Huhne, the environment spokesman, had been undermining him. He told Channel 4 News: “Those who undermine any part of the party undermine themselves as much as the party. It’s a dangerous game to play.”

Mr Huhne, who was defeated by Sir Menzies in the last leadership election, is expected to announce today that he is running again, and will soon be followed by Mr Clegg, who is the early favourite among the bookmakers and is seen by many in his party as its answer to David Cameron.

Steve Webb, who has been writing the election manifesto, is also expected to run. Charles Kennedy was considering his options, with some friends urging him to have a second try. The result of the ballot of members will be declared just before Christmas.

In his interviews Sir Menzies insisted that none of his colleagues had challenged him, although one or two “could have said different things in public”. He was clearly referring to Simon Hughes’s comment at the weekend that he would have to do better, and attacks from fringe figures.

“I had no sense that there were people wanting to move against me. If I had decided to go on and anyone had tried to move against me I would have dealt with them pretty sharply,” he said. “I took this decision. This was my conclusion, based upon my assessment and also based upon my understanding of what my responsibility to the party has got to be. I always said I would lead from the front. I always said that decisions of this kind would be for me. I thought about this long and hard. And I reached the view, without too much difficulty, that the interests of the party certainly required that I step down, and step down now.”

Asked whether he thought he would have been ejected had he not resigned, Sir Menzies replied: “I can’t speculate about the future, and it would be very foolish to do that.”

In the BBC interview Sir Menzies, who is 66, was asked if he felt relieved at standing down or frustrated at not achieving what he had set out to do.

“Irritated and frustrated,” he replied. “Irritated because of the quite extraordinary concentration on trivia which seem to surround leadership – people write articles on what kind of socks I wear. Frustrated at not getting the opportunity to lead the party in a general election, and I think our policies and our values would have been at the very centre of the political agenda.”

He stepped down after a week in which there had been “seven consecutive sets of reports about my age and about leadership”. “It became pretty clear to me, Gordon Brown having called off the election, that it was going to be very hard to get out from under that – that the sort of development of policy, the sort of presentation of policy which is necessary was going to be difficult simply because the default story in the minds of so many people is the question of my age.

“And I took the view very firmly that this was not going to be in the interests of the party and that, if I were to step down, it had to be now so that a new leader would have the opportunity of bedding himself or herself in.”

Sir Menzies said that he intended to defend his seat at the next election. “I certainly want to stay in Parliament.” Asked if he would accept a job offer from Mr Brown, he declined to answer “hypothetical questions” but added: “I would like to go on contributing.”

He also planned to finish work on a book that he had been writing before he became leader. It would provide his account of his time as leader, including the events leading to his resignation.

Leadership timetable

October 16 Nominations open

October 31 Nominations close

November 21 Ballot papers posted

December 15 Deadline for ballots to be returned

December 17 New leader will be announced

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