By JANE MERRICK and IAN DRURY

Last updated at 15:43 06 March 2008

Embattled Liberal Democrat leader Nick Clegg came under fire from the Tories today as they pledged to fight on in their battle to force a referendum on the EU Lisbon Treaty.

As Mr Clegg battles to shore up his leadership, the Opposition are now planning to reintroduce their amendment calling for a public ballot when the treaty goes before the House of Lords.

They believe the parliamentary make-up there, where the Government has no overall majority, could work in their favour even if the Liberal Democrats abstained as they did in last night's poll.

Shadow foreign secretary William Hague said: "The Liberal Democrats' position will once again be pivotal. We will see if they follow their three-line whip in the Commons to abstain."

Almost a quarter of LibDem MPs crossed the floor last night to vote for a referendum in defiance of a three-line whip imposed by Mr Clegg demanding that they abstain.

Frontbenchers Alistair Carmichael, Tim Farron and David Heath all stepped down so that they could vote with the Tories in favour of a referendum on what is the biggest shift of power to Brussels for at least a decade.

Some 29 Labour rebels also voted for the Tory amendment but the overall poll went in the Government's favour by 311 votes to 248, a majority of 63.

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A Tory amendment calling for a referendum was defeated by 311 votes to 248 - a Government majority of 63

In the Commons today, the fledgling LibDem leader, who is just three months in the job, was mocked by the Tories for plunging his party into crisis.

Shadow leader of the house Theresa May said: "Yesterday in this House the Liberal Democrat leader flunked his first big test.

"He sat on the fence as a point of principle, he divided his party after only a few weeks and he's created two classes of frontbenchers - those who have to resign for rebelling and those who don't.

"Weak, divided and vacillating. Can we have a debate on the qualities of political leadership?"

Mr Clegg was not in the chamber but his party's spokesman Simon Hughes struggled to make himself heard as Tory and Labour MPs laughed and groaned.

Sir Patrick Cormark then got in on the act by likening the Liberal Democrats to the Mugwumps - an American political movement from the 19th century.

Amidst cheers and laughter from MPs, he called for a debate on the naming of political parties.

He said: "(Ms Harman) will be aware that in 19th century America there was a party known as the Mugwumps.

"They were so called because they sat with their mugs on one side of the fence and their wumps on the other ... would that not be a better description for this lot (the Liberal Democrats) down here."

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Under pressure: LibDem leader Nick Clegg was attacked for not giving his party a free vote

Mr Clegg has denied that he made a massive miscalculation in ordering his party to abstain, claiming that he has the support of the "overwhelming majority".

But he is now under intense pressure to reassert his authority when he delivers his first big setpiece speech as party leader at the Lib Dems' spring conference this weekend.

The party has long been regarded as the most unified over the thorny issue of Europe but somehow came out of last night's poll the most bitterly divided.

Like the Government, Mr Clegg had argued that a referendum on the Lisbon Treaty was not needed because it was not the same as the failed EU constitution.

But his MPs reacted with fury, many having consistently pledged - along with both Labour and the Tories - that there would be a referendum.

None of the three MPs who resigned have been replaced, with their duties simply reassigned to other members of the frontbench team.

Mr Clegg is planning to talk to eight other rebels, including yet more frontbenchers, but the walk-out has left him with little room for manoeuvre.

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Yesterday's Commons defeat means a vote in the House of Lords - where the Government could be defeated by Tories and crossbenchers - will be the last chance for campaigners.

That vote is due later this year.

A public ballot has huge backing, with 88 per cent of voters in mini-polls last weekend demanding a chance to have their say.

The Lisbon Treaty will see the creation of a permanent EU president, foreign minister and diplomatic service and surrenders nearly 50 national vetoes to Brussels.

All three main parties made manifesto pledges in 2005 to hold a referendum on the EU constitution - now the revamped Lisbon Treaty.

Yesterday David Cameron taunted the Prime Minister that he was afraid of holding a national vote on the treaty because he knew it would be lost.

The Tory leader told Mr Brown at Prime Minister's Questions: "All of us in this House promised a referendum. We have the courage of our convictions - you have lost your courage and that lot (the Liberal Democrats) have lost their convictions."

Shadow Foreign Secretary William Hague said: "The unavoidable implication is that politicians are not trustworthy, that Parliament does not see itself as accountable and votes do not necessarily matter.

"All attempts to wriggle out of that commitment will only be seen, and will only be, the weasel words with which a solemn promise is deliberately and calculatingly broken."

Even pro-European former Chancellor Ken Clarke questioned the Government's position.

"Will you stop all this nonsense about it being different from the constitution, because it is plainly the same in substance, and explain why it is better not to have a referendum but have it decided in parliament," he said.

Mr Brown accused the Conservatives of heading towards "the margins" of Europe by failing to support the Lisbon Treaty.

He insisted that the change would not create a "United States of Europe".

Mr Clegg had argued for an "in or out of Europe" referendum, which failed to even get a vote, leading to questions over his judgment.

Insiders claimed Mr Clegg's hands were tied because the decision to abstain had been inherited from his predecessor, Sir Menzies Campbell.

But another source said the leader had failed to realise that LibDem MPs were not as sold on the European ideal as the party's peers and MEPs.

LibDem rebel Mike Hancock, a backbencher who represents Portsmouth South, described the party's position as "a mess".

He said that while Mr Clegg retained his support, the leader had blundered by not offering his MPs a free vote.

A senior LibDem MP criticised Mr Clegg for handling the issue "badly at best, incompetently at worst".

But he insisted it would not jeopardise Mr Clegg's job, saying it simply reflected his "inexperience".

However he warned: "Lessons need to be learned, and quickly."

Today, Liberal Democrat MEP Chris Davies also insisted that Mr Clegg's leadership was not under threat.

"Nick Clegg's leadership, after three months, is utterly unchallenged," he told the BBC.

"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."