They were once close allies in the European Parliament, united by talent, ambition and an education at Westminster School.

Nick Clegg invited Chris Huhne to his wedding in 2000. And even here the similarities were remarkable: both are married to bilingual wives — Mrs Clegg is a Spaniard, while Mrs Huhne is Greek — and both work as civil servants.

During those years in Europe the two men were presented as the rising stars — the modernisers who had started to make an impact even before they have arrived at Westminster — contributing to the Orange Book, in which the party’s free marketeers preached a more economically liberal direction.

But the healthy rivalry between the two men took a turn for the worse when they both arrived in the House of Commons in 2005 — Mr Clegg winning Sheffield Hallam with an 8,682 majority, while Mr Huhne won Eastleigh in Hampshire by 568.

As with so much else in the Liberal Democrats, Charles Kennedy provided the trigger for their falling-out.

Mr Clegg is understood to have been furious when Mr Huhne decided to run for the leadership in January 2006. Mr Huhne was accused of reneging overnight on a deal to step aside to give Sir Menzies a clear run. This left other young pretenders, including Mr Clegg, seething at what they saw as a betrayal.

Although Mr Huhne finished runner-up to Sir Menzies Campbell, he was popular among many of those who saw him at the hustings and had raised his profile considerably.

Nominations opened yesterday to replace Sir Menzies, and Mr Clegg and Mr Huhne are now set to take their rivalry public. The bookies put Mr Clegg in front with odds of 4-6, with Ladbrookes putting Chris Huhne on 2-1, but with the views of the 55,000 activists largely unknown it is hard to tell which way they will vote.

Last night the two camps were staking out the ground for the leadership election. Friends of Mr Clegg suggested that he was the candidate who could deliver a tough but liberal agenda, with an almost Blairite attitude towards modernising the party.

“Nick Clegg is the outward candidate and Chris Huhne is the inward one. Chris has an environmentally focused message which appeals to the base. But it is important to reach beyond the 11 per cent that already vote for us, and Nick does,” a friend said.

Mr Huhne’s supporters were highlighting the ideological differences. “Nick Clegg stands more to the Right of the party while Chris is more to the Centre Left,” a friend said.

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