Brussels warns Poland and UK on EU constitution

"Poland is currently experiencing the value of European solidarity. On a difficult issue for Poland, we clearly said: Poland's problem is a problem for the whole of the European Union," said Mr Barroso.

"I hope that Poland's leaders understand that solidarity is a two way street," he remarked, in a reference to EU support for Warsaw in its ongoing row with Moscow over Russian refusal to import Polish meat products.

Poland made the issue a European one late last year by blocking the opening of talks on renewing an EU-Russia pact, and asked other member states to show unity.

Meanwhile, the commission chief tailored the same message to London. He told UK daily the Financial Times that London would harm its own interests if it prevented agreement on a new treaty, adding that the matter would come back to haunt Gordon Brown, set to be the next UK prime minister.

"I have no doubt that this issue will not disappear unless it is solved," Mr Barroso said

"How can we have credibility in addressing global challenges when we show we are not even able to settle our internal arrangements? It's a question of credibility," he went on.

The warning - to the two countries seen as the most sceptical about a new treaty - comes just weeks before EU leaders are to meet to try and agree the bones of a new settlement, two years after the proposed EU constitution was rejected by France and the Netherlands.

Poland has said it wants to change the voting system proposed in the new constitution – a political hornet's nest that no other member state has indicated it wants to touch.

The rhetoric was upped yesterday by prime minister Jaroslaw Kaczynski. "We are ready - since there was information in the press to the contrary - to die for this," he said in a conscious echo of Poland's "Nice [treaty voting system] or Die" slogan from the 2003/2004 negotiations.

The UK, for its part, shocked other member states by recently presenting a long list of red line issues that it did not want to see in a new treaty – including the EU having a legal personality and the extension of qualified majority voting. It also wants to scrap the Charter of Fundamental Rights from the text.

Simplified treaty

Elsewhere among member states, France's idea of a "simplified treaty" appears to be gathering support.

Italy's prime minister Romano Prodi recently indicated he could live with this new set-up so long as it included key institutional reforms – such as a foreign minister, a long term president of the EU and more qualified majority voting.

Yesterday (31 May), Spain's Jose Luis Rodriguez Zapatero also endorsed French president Nicolas Sarkozy's idea after the pair's meeting in Madrid.

"We agree with a simplified, shorter treaty," Mr Zapatero said, suggesting also that he thought it possible an agreement could be reached under the German EU presidency which will run the crucial summit on 21-22 June.

Germany is hoping to extract a precise mandate at the summit. The "roadmap" would outline what issues need to be further examined and what issues from the old EU constitution will remain untouched.