Parliament is set for a huge showdown over the EU today as weeks of heated debate come to a head in a vote on Tory calls for a referendum on the controversial Lisbon treaty.

More than 30 Labour MPs are set to rebel against Gordon Brown and vote with the Tories for a public ballot in what could turn out to be the Prime Minister's first defeat in the Commons.

Tory leader David Cameron argues that all three parties must honour their promise to hold a poll on the treaty, which critics claim is practically identical to the rejected EU Constitution.

There is huge public demand for a referendum on the document, which was drawn up to replace the failed constitution after it was rejected in public votes in France and the Netherlands in 2005.

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But the Liberal Democrats may yet save Mr Brown from a humiliating defeat, after leader Nick Clegg's shock demand that his party kill off its campaign for a referendum and abstain from today's vote.

His MPs are in open rebellion at his decision to impose a three-line whip, which is almost sure to end hopes of forcing the Government to call a national vote on the treaty.

As many as 15 of the party's 63 MPs, including several named frontbenchers, are poised to defy Mr Clegg and join those voting with the Tories for a public ballot.

With the party in crisis, health spokeswoman Sandra Gidley today publicly said she will ignore the party whip - a sackable offence - by backing the referendum call.

She said: "Rightly or wrongly, my constituents all perceive I would have broken a promise I made at the last election if I do anything other than vote for a referendum on the Lisbon Treaty."

Family spokeswoman Susan Kramer yesterday reportedly stormed out of a shadow cabinet meeting in frustration over the issue, closely followed by shadow justice spokesman David Heath.

Mr Heath has already publicly stated that he also defy his leader and vote for a referendum.

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