BRUSSELS (Reuters) - The forum drafting the European Union's first constitution backed controversial proposals on Friday to allow member states to leave the EU on the condition that strict rules govern exiting the bloc.

But some members of the Convention on the Future of Europe voiced concern that introducing a so-called "exit clause" could be abused by euroskeptics and undermine the stability of the EU's common currency, the euro.

"It should be made evident that the political and economic cost of withdrawal could be very high," said Danuta Huebner, European affairs minister of future EU member Poland.

"This would not only be just, but it would be a deterrent to use the threat of withdrawal as a political weapon in negotiations within the Union," she said.

The aim of the Convention is to reform EU institutions and streamline the workings of the 15-nation bloc as it prepares to admit 10 new member states mainly from eastern Europe in 2004.

But the 105 members were unable to start debating proposals for the crucial balance of power among EU institutions which Convention president Valery Giscard d'Estaing of France unveiled on Thursday amid fierce controversy.

To assuage delegates angry that he had leaked the proposals first to the media, Giscard had promised to arrange such a debate on Friday, time permitting. But he failed to appear for Friday's plenary session, catching even vice-president Giuliano Amato off guard, and canceled his regular news conference.

A majority in the 105-member Convention said they could accept an exit clause provided seceding from the bloc was not a unilateral decision taken by one member state, but a decision taken in agreement with all member states.

Nor should a member state be forced to leave if it failed to ratify changes to the EU's governing treaties. In such a case, leaving the bloc should be voluntary, they said.

"Withdrawal should only be possible when the treaties are amended, if a member state refuses to ratify the amendments. Very strict rules and conditions must be attached," said Pascale Andreani, representing the French government.

THREAT TO EURO STABILITY?

Delegates from Germany and the Netherlands voiced concern that an exit clause could fuel constant debate on EU membership in member states where euroskepticism is strong.

"(We should not) offer this on a silver platter to euroskeptics, because they will continue to cause disruption in their national parliaments and among national public opinion whenever something difficult is proposed by Brussels," said Juergen Meyer, a representative from the German Parliament.

Such debate might not only cause political problems in the bloc but also undermine the stability of the euro, Dutch government representative Gijs de Vries said.

"Let us imagine what would happen in international financial markets if there would be a repeated or sustained debate in major member states of the Union about EU membership. What would that do for the stability of the euro?" he asked.

Representatives from Denmark and Britain, where opposition to the EU is strong, argued in favor of an exit clause, saying it would rob euroskeptics of the argument that the bloc was a prison from which there was no escape.

"Those who are concerned this will simply fuel those anti-European feelings ... will find in practice they have been proved wrong," said Gisela Stuart of the British Parliament.

"For those who use it as political card, it will be seen as calling their bluff," she added.