Jack Straw has said there will be a "problem" if France votes to reject the EU constitution - but he insisted the UK will still hold its own referendum.
The Europe debate is heating up
The foreign secretary insisted a French "no" vote later this month would not automatically kill off the treaty.
His comments came as opponents launched their campaign against the constitution, saying it would hand too much power to Brussels.
Both the Tories and Lib Dems say the government is in disarray on the issue.
The new constitution is aimed to making the EU work better now it has been joined by 10 new members mostly from eastern Europe.
It would introduce qualified majority voting on issues such as immigration and asylum, and create an EU president and foreign minister.
France holds its referendum on the treaty on 29 May.
European Commission president Jose Manuel Barroso has warned a no vote would be bad news for the French and European economies.
Mr Straw said: "If there is a no vote by France or any other member state, the EU has a problem and it will have to be considered.
"There is a formal mechanism for that to be considered if a number of countries vote 'no'."
The UK remained committed to its own policy of holding a referendum - "unless and until" the European Council decided otherwise, he said.
But he said it was in Britain's interest to ratify the treaty as it "provided for a Europe in Britain's image".
The European Council meets in June and government sources suggest a French no vote could prompt foreign ministers collectively either to abandon the treaty or have a six month "cooling off period".
Earlier, new Europe Minister Douglas Alexander sought to reassure MPs there would be a referendum regardless of the result of votes in other countries.
He quoted Tony Blair saying: "I have always said we will have a vote on the constitution... It doesn't matter what other countries do... We will have a vote on the constitution."
Mr Blair has also said there cannot be a referendum "on nothing" if the rest of Europe tore up the constitution.
Accusing ministers of confusion, Conservative shadow foreign secretary Liam Fox said: "The only way for Tony Blair to clear up this mess is to name a date for the referendum now," he said.
And Liberal Democrat spokesman Sir Menzies Campbell said a French veto would make it "political madness" for there to be a vote whose results would be meaningless.
Denis MacShane, who until this month was Europe minister, echoed that view.
He told Channel 4 News: "I don't know in Europe who thinks that if the French vote no there is a plan B. The treaty - and it's a treaty - is dead."
The No Campaign, which was launched on Wednesday, said ratifying the constitution would lead to a economic and political "crisis".
Campaign spokesman Derek Scott, a former economic adviser to Mr Blair, said: "Some of our opponents say it is a matter of being in or out of Europe. Others say that if we vote no we will be isolated.
"I think both those arguments are wrong. We are focusing on the content of the constitution, which gives too much power to Brussels."
An ICM poll for the No Campaign suggested 54% of voters opposed the constitution, compared to the 30% who backed it.
But the Britain in Europe campaign group says it is sure voters will back the constitution when the benefits of EU membership are made clear.