Britain can be "better off, more secure, more prosperous" in the EU under the terms of a new reform package unveiled in Brussels, David Cameron has said.
The Prime Minister left little room for doubt that he will campaign for the UK to stay in Europe in an in/out referendum which he said could come within "a few months", and is widely expected in June.
Changes proposed by European Council president Donald Tusk were "something worth fighting for", said the PM.
The deal, secured after months of negotiations, was good enough that he "sure would" back Britain joining the EU under these terms, if it was not already a member.
The Tusk package - which offers an "emergency brake" on migrant welfare, protections for non-eurozone states and a legally-binding assurance that the UK is not expected to pursue integration through "ever-closer union" - offered Britain "the best of both worlds" by giving it access to the single market and a voice around the table at the European Council while allowing it to remain outside the euro and the Schengen border-free area, said Mr Cameron.
But Brexit campaign group Leave.EU branded the proposals a "fudge and a farce" while Ukip leader Nigel Farage said they were "truly pathetic - No treaty change, no repatriation of powers, no ability to control our own laws, our money or our borders".
There was concern that Mr Tusk left open the question of how long any welfare curbs could remain in place, and the period for which the brake could be renewed. Britain is believed to be pushing for a seven-year period.
And in an unexpected move, he proposed that in-work benefits for EU migrants should be phased in gradually over a four-year period while the brake is in operation, rather than being banned outright as the PM wanted.
Under the proposals, EU states could apply to use the mechanism if "exceptional" levels of migration are harming their social security system, jobs market or public services.
In a key concession to Mr Cameron, the European Commission issued a declaration that the UK already meets this threshold. But the lengthy process of introducing necessary regulations could delay the implementation of the brake in Britain until 2017 at the earliest.
The publication of the Tusk proposals kicks off an intensive period of negotiation with the other 27 EU states ahead of a crunch European Council summit on February 18-19, starting with a visit to Poland and Denmark by Mr Cameron on Friday.
Downing Street stressed that the whole package remains open to negotiation with other member-states. Failure to win unanimous support from all 27 would almost certainly delay the referendum until after the summer.
But Foreign Secretary Philip Hammond indicated that stiff resistance was not expected, telling Sky News: "I would be very surprised if we have significant negative reaction across the EU to the text that's been tabled."
Speaking in Chippenham, Wiltshire, Mr Cameron said Britain could survive and succeed outside the EU and acknowledged that the EU will not be "a perfect and unblemished organisation" after the implementation of the reforms.
But he added: "I think we will be able to show - if we can secure what's in this document, finish off the details and improve it still further - that on balance Britain is better off, more secure, more prosperous, has a better chance of success for all of our families and all our people inside this reformed European Union."
Mr Cameron said the Tusk document addressed four issues that "go to the heart of what we need to fix".
He added: "We want to have a Europe where we are not subsumed into a superstate but we can be proud and independent. We want a Europe that is competitive, we want a Europe that respects our currency and treats us fairly and we want a Europe that takes the pressure off in terms of migration."
Despite critics' predictions that he would not be able to achieve his negotiation goals, he insisted that all of the areas of key concern had been "addressed in a proper way" and that he could say "hand on heart" that he had delivered on commitments in the Conservative election manifesto.
In a letter to EU leaders, Mr Tusk said the package was "a good basis for a compromise", adding that "there are still challenging negotiations ahead. Nothing is agreed until everything is agreed".
As expected, the document proposes measures to improve EU competitiveness, a 55% threshold for national parliaments to force the EU to alter or scrap proposed laws, as well as assurances that non-euro states are not required to help bail out single currency members.
And it proposes a new mechanism for them to escalate concerns about possible discrimination in favour of the eurozone for discussion by the full European Council, as the Prime Minister has demanded.
The document states in law for the first time that the euro is not the EU's only currency and that the commitment to "ever-closer union" does not oblige all member states to "aim for a common destination".
In a key gain for Home Secretary Theresa May - tipped as the most senior Cabinet minister still weighing up whether to campaign for Brexit - the document makes clear that EU states are entitled to refuse access "on preventative grounds" to individuals likely to threaten national security, even if the threat is not imminent.
The chairman of the Labour In For Britain campaign, Alan Johnson, said his party was pleased that "the starting pistol has been fired" and was hoping for a referendum "the quicker the better", ideally on June 23 or 30.