BRUSSELS (AFP) -- European leaders failed to make any headway at a summit designed to thrash out a first-ever EU constitution, with the prospect for reform of the enlarging bloc's institutions fast receding.

After a first round of formal talks, the summit chairman, Italian Prime Minister Silvio Berlusconi, abruptly canceled a planned joint dinner and cloistered himself in a series of head-to-head talks late into the evening.

Berlusconi was expected to present new compromise proposals to his colleagues early on Saturday, which an Italian source said could tinker with the voting arrangements in an enlarged EU.

But criticism mounted of the maverick billionaire's high-wire style of negotiating.

"We haven't got anywhere. And I'm very skeptical about the possibility of getting an agreement on a text," said Luxembourg Prime Minister Jean-Claude Juncker, clearly exasperated. An EU minister who declined to be named said: "If Berlusconi has nothing to propose tomorrow morning, we should pack up. It's pointless."

The summit -- scheduled to last two days but which is shaping up to be a marathon -- aims to streamline the EU's decision-making process after it expands from 15 to 25 members next May, and further in 2007.

Among the disputes are the composition of the executive European Commission, the powers of a new EU president and whether the constitution should refer to Europe's Christian heritage.

But the key sticking point is national voting rights. Spain and Poland are refusing to cede the disproportionate influence they secured at a chaotic EU summit in Nice three years ago.