BRUSSELS (BBC)- European Union leaders have failed to reach agreement on a constitution at a crucial summit in Brussels.

The talks were deadlocked by Poland and Spain's refusal to surrender voting rights secured at a summit in Nice three years ago.

The EU expands from 15 to 25 states next year, and agreement at these talks was seen as vital.

The leaders are now preparing to leave Brussels after two days of sometimes bitter exchanges.

BBC correspondent quotes one official as saying that France forced the breakdown of talks by refusing to consider any compromise on the voting issue.

He adds that France and Germany are now expected to make a joint statement about the future of the talks on the constitution - and the future of the European Union itself.

The Nice agreement on EU expansion gave Spain and Poland - one of the new members - almost as many votes each as Germany, despite them having smaller populations.

Warsaw and Madrid say the subsequent constitution proposals significantly diminish their power, and are unacceptable.

Saturday saw Italian Prime Minister Silvio Berlusconi - as host of the talks during Italy's tenure of the EU presidency - present four alternative proposals to Spain and Poland in an attempt to break the deadlock.

But Polish Foreign Minister Wlodzimierz Cimoszewicz signalled his country's refusal to give ground on the voting rights issue.

"If it's not possible to agree, we shall wait. We're talking about compromise or domination," he said.

All 25 leaders must approve the proposed constitution.

The constitution, drafted over 17 months by a special convention, introduces a "double majority" system of voting.

It means a vote is passed when it has the support of 50% of countries, representing 60% of the EU's population.

Medium-sized countries like Poland and Spain say the system favors big states - like Britain, France and Germany - and the very smallest nations.

The problem now passes to Ireland, which takes over the EU presidency from 1 January.