[BRUSSELS] EU president Donald Tusk unveiled proposals to keep Britain in the 28-nation club on Tuesday, firing the starting gun for two weeks of tense negotiations to reach a deal at a summit later this month.
British Prime Minister David Cameron said Mr Tusk's plans showed "real progress" and made it likely that he would campaign to stay in the European Union in a referendum expected in June.
The proposals include a four-year "emergency brake" on welfare payments for EU migrant workers, protection for countries that do not use the euro currency and a "red card" system giving national parliaments more power.
But eurosceptics in Britain dismissed the proposals as worthless, and they could be a hard sell ahead of the February 18-19 summit for some EU states who fear that Mr Cameron is winning too many concessions.
"To be, or not to be together, that is the question which must be answered not only by the British people in a referendum, but also by the other 27 members of the EU in the next two weeks," Mr Tusk said in a letter to EU leaders.
Mr Tusk later warned that a deal was not certain in the pre-summit negotiations, which will begin in earnest on Friday when EU diplomats meet in Brussels.
"It's still a lot of work ahead of us. The stakes are really high," Mr Tusk told the BBC. "Nothing is easy in this case."
US President Barack Obama waded into the debate on Tuesday, calling Mr Cameron to reiterate his support for "a strong United Kingdom in a strong European Union", according to the White House.
Washington has long backed Britain playing a central role in the world's largest economic bloc.
London's bid to transform its EU membership has added to the turmoil as the bloc struggles with its worst migration crisis since World War II and the fallout from the eurozone debt saga.
As part of a charm offensive, Mr Cameron will visit Poland and Denmark on Friday then Germany next week for talks with Chancellor Angela Merkel.
The British premier said Mr Tusk's proposal showed that he had "secured some very important changes".
"If I could get these terms for British membership I sure would opt in for being a member of the EU," Mr Cameron said in a speech in southwest England.
Initial European reaction was muted but Czech Europe minister Tomas Prouza said the welfare brake plan was "acceptable" - a positive sign given recent criticism by central European countries that the scheme would discriminate against the hundreds of thousands of their workers in Britain.
But British eurosceptics were unconvinced, with London Mayor Boris Johnson, from Mr Cameron's own Conservative Party, saying he had "doubts" about the effectiveness of the "red card" proposal.
UK Independence Party head Nigel Farage dismissed Mr Tusk's proposals as "pathetic" and "hardly worth the wait".
Tusk's most controversial proposal is an "emergency brake" that would allow any EU state to limit the welfare payments that migrants from other European countries can claim for up to four years after their arrival.
States would have to prove an "exceptional situation" in which their welfare system and public services are overwhelmed - but instead of Mr Cameron's demand for an outright ban it said such limitations should be gradually reduced over the four years.
To pull the brake, a country would also have to get approval from the European Commission, the powerful executive arm of the EU, and then from other EU leaders in a majority vote.
Despite concerns in France, Mr Tusk's plan also includes a "mechanism" by which the nine countries that are not in the euro can raise concerns about decisions by the eurozone.
But he stressed that the mechanism could not delay or veto urgent decisions by the 19 euro countries.
Britain will be further exempted from the EU's stated goal of "ever closer union" because of its "special status" in the bloc's treaties - including staying out of the euro and the passport-free Schengen area.
The "red card" system would allow a group representing 55 per cent of the EU's national parliaments to stop or change draft EU laws.
Although Mr Cameron has only set a deadline of end-2017 to hold the referendum, sources have said he is keen to push a vote through by June.
That would avoid the fallout from any new flare-up in Europe's migration crisis this summer and British eurosceptic elements becoming even more unruly.
Opinion polls are split on whether Britons would back leaving the EU in their first vote on the subject since 1975.