Ken Clarke, the shadow business secretary, admitted that a Conservative government would not try to renegotiate the EU's Lisbon Treaty if it was ratified in an Irish referendum, causing outrage among Eurosceptics in his party.
The comments from Mr Clarke, whose appointment to the post was greeted with hostility by some of the party faithful because of his pro-Europe views, were seized on by David Miliband, the foreign secretary, as showing the Tories in "disarray" over Europe.
Mr Clarke said yesterday that the shadow cabinet's "settled position is quite clear that the treaty will not be reopened" if Ireland backs the agreement.
David Cameron, the Tory leader, has said that he would call a referendum on the Lisbon treaty if his party won a general election before it was ratified by all EU member states.
However, when pressed on what he would do if the Irish voted "yes" and the treaty was subsequently agreed across the EU, he has only responded cryptically that he would "not let the matter rest there".
Mr Clarke's comments yesterday, in an interview with the BBC, are the first time that the party has stated openly that any further measures taken in the event of an Irish "yes" vote would not include a UK referendum on the Treaty.
Instead, Mr Clarke said the Tories would seek negotiations with the EU "about the return of some responsibilities, particularly in employment law to individual nation states . . . I think there are some other member states who think it's perfectly legitimate to start reconsidering whether or not something like the opt-out to the old social chapter might not be reconsidered."
Irish voters rejected Lisbon in June last year but EU leaders have been discussing legal guarantees on taxation, neutrality and ethical issues to pave the way for a second referendum, probably in October.
Mr Clarke said guessing the outcome of an Irish referendum was "reckless" but opinion polls show the Yes camp leading the No camp by a 54 to 28 per cent margin.
Bill Cash, the veteran anti-Europe Tory MP, condemned Mr Clarke's statement.
"It is essential we have a referendum irrespective of the Irish vote and this is a view supported by a substantial number of Tory MPs," he said.
"It appears he has unilaterally reinvented Tory party policy."
Mr Miliband said: "The country deserves a clear answer: has Conservative policy flip-flopped, or are they just divided and incredible in their foreign policy?"
The Conservatives insisted there had been "no change" to their policy.
Nigel Farage, leader of the UK Independence Party, said: "Ken Clarke has just let the cat out of the bag.
"The Conservatives have no intention of holding a referendum on the Lisbon Treaty."
