A leading Liberal Democrat leapt to the defence of the party leader Nick Clegg today after three Lib Dem frontbenchers defied a three-line whip to vote for a referendum on the European constitutional treaty.

The resignation of the three senior spokesmen, David Heath, Alistair Carmichael and Tim Farron, represented the first crisis of Mr Clegg's brief leadership of the party. They were among 15 of 63 Lib Dem MPs who voted with the Tories for a referendum on the Lisbon Treaty, ignoring party orders to abstain, although the Government still defeated the amendment by 63 votes.

The reverse is bound to overshadow the party’s spring conference in Liverpool this weekend, Mr Clegg’s first important party event since he became leader in December.

Chris Davies, the Liberal Democrat MP for North West England, inisisted today that Mr Clegg's leadership of the party - he took over from Sir Menzies Campbell in December - was not under threat and dismissed the referendum debate as "incredibly trivial".

“Nick Clegg’s leadership, after three months, is utterly unchallenged,” he told BBC Radio 4’s Today programme.

“The man is so obviously intelligent, so disarmingly good-humoured and self-deprecating even in situations like this and so absolutely determined to fight the liberal corner even when his views may not be popular, that he is at one with the very heart and soul of this party and you will see that at the weekend’s conference.

“Nick Clegg is going to be leader of the Liberal Democrats for a very long time.”

He added: “He has had a very difficult set of cards to play but an abstention is not a cop-out. It is just a third choice when the other two are not acceptable.”

Privately many MPs, including those supporting Mr Clegg’s position, wondered why he was picking a fight on the issue when he could have allowed the party a free vote.

By ordering his MPs to abstain he got the worst of both worlds: criticism for apparently weak leadership but a revolt all the same.

MPs complained that if he had allowed a free vote his MPs from Euro-sceptic constituencies could have opted for a referendum and avoided local criticism, while the majority would still have opposed a referendum and helped the Government to get its way.

Last night Mr Clegg sought to play down the revolt, insisting that he had the support of the “overwhelming majority” of the parliamentary party.

“These things happen on issues with the importance of Europe,” he told Channel 4 News. “It is not such a big thing that from time to time in a parliamentary party there is a division of opinion.”

He said that he would be speaking to a number of junior spokesmen who joined the revolt in the coming “hours and days” but appeared to rule out further sackings.

“Do I reassert my authority by having some great cull? This is not the way that politics works. I will make sure – have made sure – that there is discipline in the overwhelming majority of the parliamentary party,” he said.

According to one Lib Dem MP the rebellion was sealed after unsuccessful meetings at Mr Clegg’s Commons office when the trio opted to resign rather than be sacked. Even the three frontbenchers told Mr Clegg that they believed he should sack them.

Lib Dem sources insisted last night that the process had been “amicable” and came after lengthy discussions that established the party’s position.

Mr Clegg only insisted on resignations from members of his shadow ministerial team. Other frontbenchers faced a rap from the whips.

All three senior spokesmen are likely to return to the front bench after an interval. For now, Chris Huhne, the home affairs spokesman, will take on justice, Steve Webb’s environment team will take on the countryside brief and Michael Moore, the international development spokesman, will take on Scotland and Northern Ireland.

Mr Clegg said: “I am grateful for their offer of strong support from the back benches, and I know that they will all play a very significant role in the future of our party. I’m not banishing people to outer Siberia.”

Earlier in the Commons, Mr Clegg, who became leader after the short reign of Sir Menzies Campbell, was mocked by all sides. Michael Ancram, a former Tory minister, said: “This debate is going to be remembered most for the impressive sight of the Liberal Democrat Party marching, with sound and fury, courageously towards the fence upon which they have been ordered to perch.”

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