The body drafting a constitution for the European Union - the European Convention - has proposed an unprecedented exit clause for members.
Giscard says Iraq row will delay the constitution's completion
The clause would allow member states to leave the union if they give two years' notice.
It would be the first such clause in the EU's history, and countries which have traditionally been euro-sceptic have welcomed it.
The British representative, Peter Hain, called the proposal "excellent".
He warned that it if the clause were rejected, this would strengthen arguments against the EU.
But opponents of the move say it could further deepen divisions over the constitution.
"There is a growing distrust between large and certain small countries. This does no good to our work," said convention deputy chairman Jean-Luc Dehaene, a former Belgian prime minister.
The draft clause - which if approved would become Article 46 of the future treaty - will be submitted to the next full session of the convention at the end of April.
Under the clause, any request to opt out of the EU would have to be approved by the European parliament the council of ministers.
Once this was completed, the state would no longer participate in the council and would no longer be bound by the EU constitution.
The convention is hoping to finish drafting the constitution by June, but its president, the former French President Valery Giscard d'Estaing, has said the war in Iraq is likely to delay its completion.