David Cameron has urged EU leaders to seize a once-in-a-generation opportunity to resolve the "festering" problem of Britain's relationship with Europe, by giving him a "credible deal" he can sell in the upcoming referendum.
The Prime Minister's plea came as other EU states demanded a "no second chance" agreement to make clear to Britain that Europe will not come back with a better offer if it votes to Leave.
As wrangling over a proposed reform package got under way in earnest at a "make or break" Brussels summit, Britain's line on key welfare measures appeared to be hardening.
It is understood the PM has set his face against calls from eastern Europe for child benefit cuts to be imposed only on new migrants with offspring living in their home country.
Mr Cameron wants the new system - under which payments would be made at the lower rates of the migrant's homeland - brought in quickly and believes it is not acceptable for it effectively to be phased in over 16 years as the children of 34,000 existing claimants in the UK reach adulthood.
UK officials said child benefit was among a number of key issues, including protections for non-euro countries and a proposed emergency brake on migrant welfare, which remain to be settled at a crunch European Council summit.
Mr Cameron was prepared to keep talking into the night as officials try to hammer out the precise terms of a deal which can be agreed on Friday, clearing the way for the announcement of a referendum date following a special Cabinet meeting in London.
Belgium - backed by France - moved to pre-empt the possibility of a second UK referendum by proposing that the summit conclusions should state that any agreement reached this week will not be amended if Britain votes to leave the EU.
The move is designed to quash the idea, backed by some Eurosceptics and reportedly floated privately by London Mayor Boris Johnson, that a Leave vote would give the UK leverage to extract further concessions from the EU before a second poll.
Mr Cameron's characterisation of the package drawn up by European Council president Donald Tusk as a once-in-a-generation moment suggests he will not seek to block the Belgian initiative.
Addressing fellow leaders at the start of the two-day summit, Mr Cameron said the question of Britain's relationship with the EU had been "allowed to fester for too long" and it was time to deal with it.
And he told them he needed a package that would be "credible" with the British people and strong enough to persuade them to vote to remain in the EU.
The 28 leaders at the European Council had the chance to "settle the issue for a generation" and move to a "fundamentally different" relationship with Britain, he said.
The new arrangement on offer would be a "live and let live" settlement under which states which want to integrate further will be free to do so, while those which do not can rest assured their interests will be protected, said Mr Cameron.
This would be "a big prize" for every nation in Europe, he told his fellow leaders.
Arriving at the Brussels summit, Mr Cameron said he was still ready to walk away without agreement if the package on offer did not meet UK concerns.
Declaring he was "battling for Britain", he said: "If we can get a good deal I will take that deal but I will not take a deal that does not meet what we need."
A number of EU leaders - including German Chancellor Angela Merkel - expressed their readiness to seek an arrangement that would allow Britain to stay in the 28-nation bloc.
But French President Francois Hollande left no doubt there were limits to the compromises he would accept.
"No country must have a right of veto, no country must exempt itself from the common rules or common authorities," said Mr Hollande.
"It's the European Union that's at stake, not simply one country of the European Union.."
Mr Tusk - who has spent the last week on a whirlwind tour of European capitals in search of consensus - described the meeting as a "make-or-break summit".
But European Commission president Jean-Claude Juncker said he was confident outstanding differences would be resolved and "convinced" the UK would remain a "constructive and active" member of the EU.
Ukip leader Nigel Farage dismissed the package on offer as "pretty shameful", telling the Press Association: "Whatever he comes back with and tries to sell to the British people is not legally binding. It can be struck down by the European Parliament and ultimately all of it can be ruled out of order by the European Court of Justice.
"It's rather like him saying to the British people, 'I'd like you to buy this car, but you can't see whether the engine works first'. It just doesn't work."
And Labour leader Jeremy Corbyn, meeting fellow European socialists in Brussels, said Mr Cameron had "brought an internal Conservative Party dispute to international proportions".
"They may well end up with some kind of agreement which he will then present as a victory," said Mr Corbyn.
"The reality is he should face up to the situation - does he think Britain should remain in Europe or not? Is he negotiating for better working conditions and social protection across Europe or not? No he isn't, he's doing the exact opposite of that."