EU leaders head into a make-or-break summit Thursday sharply divided over difficult compromises needed to avoid Britain becoming the first country to crash out of the bloc.

The 28-member EU is also battling its worst migration crisis since World War II and has proved unable to forge a common response to waves of people fleeing conflict in Syria, the Middle East and North Africa.

European Commission head Jean-Claude Juncker was tight-lipped when reporters asked if he would take a question about what promises to be a tense two or even three-day meeting.

"No, not one," Juncker said.

British Prime Minister David Cameron, under pressure from eurosceptics in his own party and a hostile right-wing press, has demanded a series of reforms that will return powers to London.

But the British position has exposed a dramatic rift over the EU's future -- whether it should continue as an "ever closer union" or become a much looser group of distinct sovereign nations.

Four Eastern European states oppose Cameron's call for a limit on welfare benefits to EU migrants working in Britain, while France leads opposition to protections for countries like Britain that do not use the euro.

In a demonstration of the likely battles ahead, a leaked draft of the summit conclusions still had a number of key passages in brackets, including on the "euro-outs" and on migrant benefits, meaning they have not been agreed despite weeks of tense negotiations.

If the summit fails to clinch a deal, Cameron has said anything is possible, including Britain becoming the first country to quit the bloc after an in-or-out referendum that could be held in June.

EU President Donald Tusk warned late Wednesday there was "no guarantee" of an accord.

In London, a UK government official said the prime minister had taken "a very personal involvement" in the talks and now "this is crunch time".

"We think we have made a lot of progress and we will be going into this summit to nail down the rest of the details," said the official who asked not to be named.

"We do think we're in a good place."

Cameron has staked his political survival on winning the vote in the hope of ending a feud over Britain's place in the EU that has plagued his Conservative Party for decades.

Britons voted overwhelmingly in favour of staying in the EU in a 1975 referendum, just two years after joining, when the then Labour government said the country would miss out on Europe's growing prosperity if it left.

That is the same case made now but with Britain more prosperous and growing faster than most of its EU peers, eurosceptics say 'Brexit' should hold no fears.

"My guess is that there will be an agreement but this is so far away from the fundamental reform as to be quite frankly pitiful," Nigel Farage of the eurosceptic UK Independence Party told AFP.

Cameron has four key demands -- welfare restrictions to help curb immigration, safeguards for non-euro Britain, increasing EU competitiveness and an opt-out from closer EU integration.

The key sticking point is that EU citizens working in Britain should not have access to welfare benefits for four years.

Poland and other east European member states who have hundreds of thousands of citizens in Britain bitterly oppose such a change, saying it would discriminate against them and undercut the EU's core principle of freedom of movement.

Brussels has offered an "emergency brake" to limit benefits for new migrants for four years after their arrival, which Britain could invoke if its welfare system is overwhelmed by the inflow of workers, as it believes it has been.

The right-wing British press was dismissive, plastering their front pages Thursday with reports the number of EU workers in Britain had grown to two million.

"2M EU migrants grab our jobs" was the headline of the eurosceptic Daily Express while The Times described the figures as a "hammer blow" to Cameron.

Cameron won crucial backing, however, from German Chancellor Angela Merkel, Europe's power-broker who said Berlin had shared his EU concerns "for many years."

Elsewhere, support is lacking or lukewarm.

On Cameron's call that non-euro Britain have safeguards against closer integration of the single currency area, France insists that Cameron must "in no circumstances" get a veto over the eurozone to favour the City of London financial hub.