Labour has accused Conservative leader David Cameron of risking years of uncertainty and instability for the UK by suggesting he would not accept the implementation of the European Union Treaty of Lisbon.
The treaty, signed by Prime Minister Gordon Brown and other EU leaders earlier this month, is expected to come into force on January 1 2009 after being ratified by all 27 member states during 2008.
Only Ireland, which is constitutionally bound to, has so far said it will hold a referendum, while the other 26 members are expected to ratify the document by a vote in their parliaments.
But Mr Cameron said that the Conservatives - who are campaigning for a referendum on ratification in the UK - would not be content with this situation and "wouldn't let matters rest there".
Europe minister Jim Murphy claimed that the Tory leader's comments appeared to commit his party to holding a vote on the treaty even after it has been ratified.
Speaking to the Daily Mail, 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."
Mr Cameron did not spell out what action he would take if the treaty was ratified by the UK and implemented, and he stopped well short of explicitly committing himself to a post-ratification referendum, which some in his party want.
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But Mr Murphy said: "Unless they tell us otherwise, it appears Cameron has committed the Tories to a vote on the EU treaty even after it has been ratified by all 27 countries of the EU. The Tories' position would lead ultimately to the disaster of Britain's withdrawal from the EU, and in the meantime would be a recipe for years of uncertainty and instability that would be damaging to British business, British jobs and British prosperity."
