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Tory leader David Cameron faced a furious backlash from his own MPs yesterday as he shamelessly broke his pledge to hold a referendum on Europe.

Eurosceptic Tory MPs rounded on their leader after he said he would not keep his "cast-iron" promise to give voters a say on the Lisbon Treaty.

Mr Cameron's humiliating U-turn came after Czech President Vaclav Klaus became the last EU leader to sign the Treaty. The historic move, eight years after the plans were first raised, will streamline the 27-strong bloc's business.

The Tory leader gave his guarantee in 2007 saying: "If I become PM a Conservative government will hold a referendum on an EU treaty." He claimed failure of politicians to keep promises was the "cancer eating away at trust in politics". But yesterday he tried to squirm out of his pledge by arguing once Lisbon became law it was no longer a treaty - then sent William Hague out to face the cameras.

Mr Cameron, who may now try to take back powers from the EU instead, said he will set out the new Tory position today - leading to accusations he was burying bad news on the day the Kelly report into MPs' expenses is published.

His double-talk sparked fury among Tory MPs and MEPs. Nearly 50 backbenchers have signed a Commons motion demanding a referendum "before or after ratification".

Conservative MP Richard Shepherd said Mr Cameron's honour was on the line. "It's a matter of principle, honour and trust. The political class seems indifferent to allowing the people to express their own view on something as profound as the development of Europe," he said. Business Secretary Lord Mandelson said: "Cameron's cast-iron guarantee has become very rusty indeed."

The Czech's signing puts pressure on Tony Blair to decide whether he wants to run for the new post of EU president though sources close to the former PM said he had still not made up his mind.

Gordon Brown hailed the ratification as an "important and historic step for all of Europe" and said he would back Mr Blair if threw his hat in the ring.

Foreign Secretary David Miliband said of the Cameron U-turn: "He is still not being honest with people. The fact is you can't simply opt out of treaty obligations because to do so you need the agreement of the 26 other member states.

"If, in the end, the Tories failed to get such an agreement but remained committed to such repatriations, a Conservative Britain would have to withdraw from the EU.

"He is willing to risk Britain's standing because he is still not prepared to stand up to the Right of his own party."

Lib Dem spokesman Ed Davey said: "He must come clean on where he now stands. He cannot bleat about transparency then go awol as soon as there are tough questions to answer."

Q&A

What is the Lisbon Treaty?

An attempt to make the EU "efficient and transparent" now it has 27 members. It also creates a President and a foreign chief to represent the EU internationally.

What else changes?

The number of Euro Commissioners, the people with the real power, is cut. It will be harder for countries to veto directives. Most decisions will be decided by majority voting rather than requiring the backing of all members. A bigger role for the Court of Justice. Euro Parliament will have more powers to scrutinise policies.

How will it affect us?

Not much at first - the changes are over 10 years. EU may have more say in foreign affairs but Britain insists its right to set its own laws on tax, social security and defence.

Why did yesterday matter?

The Czechs became the final country to sign up, meaning Treaty can now become law.

Why is that a problem for David Cameron?

He promised in 2007 he would hold a referendum on the Treaty if he became PM.

So why not hold one?

It would be impossible to reopen the Treaty without risking Britain being kicked out of Europe. There's no guarantee he would win a referendum if it became a vote on Britain staying in EU. Last thing he wants is for a Euro row to dog first six months of his government.

So what's he done?

He's trying to squirm his way out of it by claiming once Lisbon becomes law it is no longer officially a treaty. Many Tory MPs say this won't wash and he should honour his commitment.

What about Tony Blair?

He may become EU President but Germany's Angela Merkel is said to be cool on the idea. Foreign Secretary David Miliband is linked with the EU foreign affairs job.