The race for the Liberal Democrat leadership was taking shape tonight as Nick Clegg and Chris Huhne declared their interest in the job.

Mr Huhne, the party's environment spokesman, was the first confirm he would stand as a successor to Sir Menzies Campbell this afternoon a few hours before Mr Clegg’s campaign team said his bid would be formally launched on Friday.

The acting party leader, Vince Cable, and its transport spokeswoman, Susan Kramer, both ruled themselves out as candidates.

This lays the ground for a two-horse race between two men who have risen rapidly through the Lib Dem ranks. After careers in financial journalism, they were both elected MEPs in 1999 before graduating to the House of Commons in 2005.

Bookmakers have installed the home affairs spokesman Mr Clegg as the favourite, but his opponent has more experience of these contests after finishing as the runner up to Sir Menzies in last year’s leadership election.

Speaking at a Westminster restaurant, Mr Huhne said that he would be campaigning for a “fairer, greener society where we put people in charge”.

John Hemming, the Birmingham Yardley MP, is also considering a run as a potential “dark horse” candidate.

Mr Cable, the party's treasury spokesman, said that he had come to the conclusion that an older candidate was unelectable because of the of the “irrational prejudice” shown towards Sir Menzies over his age. At 64, he is just two years younger than Sir Menzies.

Dr Cable represented his party at Prime Minister's Questions today, his first Commons appearance since Sir Menzies's abrupt resignation on Monday afternoon.

MPs heard Gordon Brown, the Prime Minister, pay tribute to Sir Menzies, a fellow Scot and long-time friend, as a man of decency and integrity before he quipped: “If things go on in this way, every single member will have a chance of being leader of the Liberal party.”

Charles Kennedy, who quit as Lib Dem leader in January last year after admitting to a serious drink problem, said today that "thousands" of potential supporters had contacted him since Sir Menzies's resignation.

But asked on BBC Two's The Daily Politics whether he would bid to return to the top job, he replied: "I think it's highly unlikely, to be honest."

He added: "You should never say never in politics, but as close to never as you can get."

In broadcast interviews yesterday, Sir Menzies explained for the first time why he had decided to quit as party leader - blaming media obsession with his age rather than any party plot against him.

The Scot said that he had felt "irritated and frustrated" by constant press speculation about his leadership, but it was not until the Prime Minister finally decided against calling an early election that he had decided that the age issue was not going to go away.

"There were seven consecutive days in which there were seven consecutive stories or reviews about my age and it became very clear to me it was going to be very hard to get out from under that,” he said.

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