European Commission President Romano Prodi has said a two-speed Europe might be inevitable if EU leaders fail to agree on a new constitution this year.
Talks on the new EU charter broke down at a summit last month with members unable to agree on future voting powers after the 15-member bloc expands to take in 10 new members.
"It is clear that if the situation does not unblock in 2004 then some (countries) could, and perhaps should take the initiative to go forwards," Mr Prodi said in an interview with Italian newspaperLa Repubblica today.
"They might be the founder members. Or even, and this is more likely and more desirable, a mixed group of old and new states that share the same view of Europe," he said.
"The Union train cannot always move at the speed of the slowest wagon. In fact, I have the impression that some of the wagons don't want to move or even want to go backwards."
France and Germany, blocked in their bid for more voting power by Spain and Poland, have said they could lead "pioneer groups" of like-minded countries towards closer integration, raising the prospect of a two-speed Europe.
Other countries have urged restraint, fearing such a move could lead ultimately to the break-up of the European Union.
The 10 new countries, mainly ex-communist states from eastern Europe, are due to join in May.