The prospect of Russia eventually joining the EU was held out by Liberal Democrat MEP Graham Watson at a fringe meeting organised by the European Commission.

Mr Watson, who is leader of the British Lib Dem group in the European Parliament, told the meeting that enlargement of the union was the biggest challenge currently facing the EU.

"The EU is already in a process of enlargement that will fundamentally change the nature of the union," he said.

Some of the countries from Eastern Europe already bidding to join had national economies far weaker than the average within the EU and would need massive amounts of help to bring them in line with the rest of the group, he said.

But, beyond the admission of Eastern European countries, there was the longer term possibility of many other countries, including Russia joining the EU, he said.

No courage

And he attacked Western European governments for showing a lack of courage by refusing to go out and debate the issue with their voters.

"There is insufficient discussion and debate in Western Europe about what enlargement means.

"Governments do not have the courage to go out to their electorates and explain what it is all about.

"This is one of the greatest challenges to us - to start that debate," he said.

He also warned that the Liberal Democrat influence in the EU would be weakened by the inclusion of countries that did not have a liberal tradition.

But he insisted the party had a duty to "take the bull by the horns" and raise the debate about the issue.