David Cameron: I'll tear up the EU treaty even if it has been signed

The Conservative leader told the Daily Mail he will "not let matters rest" if Gordon Brown succeeds in forcing the controversial treaty through Parliament and into law.

His intervention ratchets up the pressure on Mr Brown over the document which is likely to dominate debate at Westminster in the New Year.

Ministers are hoping to bore voters into submission by allowing weeks of lengthy discussion on the treaty.

But Conservative MPs and a band of Labour rebels are promising the biggest parliamentary showdown over Europe since the Maastricht Treaty plunged John Major's government into turmoil in the 1990s.

Mr Brown plans to use Labour's Commons majority to push the treaty into British law despite a manifesto promise at the last election to ask for voters' approval in a referendum.

The Prime Minister insists that the constitutional element of the treaty has been abandoned, making a vote unnecessary.

But most other EU leaders admit that it is virtually the same as the original version, which was rejected by voters in France and the Netherlands in 2005.

It will still create the first full-time EU president and foreign affairs chief, give the EU its own legal personality like a nation state, and do away with Britain's right to reject EU proposals in more than 40 policy areas.

Mr Cameron is already the only party leader to have promised to honour a manifesto commitment to hold a referendum on the document if he wins power before it has been implemented.

Labour, the Conservatives and the Liberal Democrats all promised a vote on the Constitution in the 2005 election campaign.

But so far the Tory leader has resisted giving any firm pledge on what the Tories would do if the treaty has already been forced into law by the time they take office.

Euro-sceptic Tory MPs have insisted there should be a referendum even if the treaty is already in place.

But attempting to tear up a treaty after it has been agreed and made law across Europe would be fraught with difficulty and will put Britain on a collision course with other EU countries.

Mr Cameron said: "While this treaty is still being debated and other countries are having referendums or whatever, it's still open for Britain to have a referendum. Let's focus on the need for a referendum now.

"If we reach circumstances where the whole treaty has been not only ratified but implemented that is not a situation we would be content with.

"We wouldn't let matters rest there.

"We think the treaty is wrong because it passes too much power from Westminster to Brussels.

'We would address that issue at the time.'

Labour ministers will seize on Mr Cameron's remarks as evidence that he is prepared to renegotiate Britain's entire relationship with the EU.

His commitment to unpick the treaty after it has been implemented could even lead to Britain being forced to leave the EU altogether, they will claim.

But the strength of Mr Cameron's pledge will delight his largely Euro-sceptic back benchers and traditional supporters.

And a hardcore of around 30 Labour MPs is promising to rebel.

They argue the government's promise of full parliamentary discussion of the treaty is meaningless without a free vote that would allow MPs to vote as they wish.