A draft deal on his reform demands delivers the “substantial change” he wants to see to the UK’s relationship with the EU, David Cameron has said.
But the UK prime minister said there was “detail to be worked on” before a crunch summit on 18-19 February.
The deal, published by European Council President Donald Tusk, allows for an “emergency brake” on migrant benefits.
But Leave EU campaigners said the deal did not come close to the changes Mr Cameron had promised.
What’s in the draft deal
Mr Cameron’s proposed four year ban on in-work benefits for EU migrant workers could come into force immediately if the UK votes to remain in the Union.
But it would have to be agreed by other EU nations and it would be “graduated”, with more money from tax credits paid to migrants the longer they remain in the UK.
It says Mr Cameron’s demand to exempt Britain from the EU principle of “ever closer union” between member states would be written into a future treaty.
There are also measures relating to protection for non-euro countries in the EU, a new way for member states to club together to block some new EU laws and on business regulations.
What happens now
Mr Cameron will visit Poland and Denmark on Friday, as he embarks on a whirlwind charm offensive to persuade the other 27 EU leaders to sign up to the Tusk package in Brussels on February 18-19.
If Mr Cameron can get an agreement in February, he is expected to hold a referendum in June on whether Britain should remain in the EU.
Mr Cameron has until the end of 2017 to hold a referendum. A July or September referendum remains a possibility but a repeat of last summer’s migrant crisis in the Mediterranean and eastern Europe could make Mr Cameron’s job of making the case for remaining in a reformed EU more difficult.
A cross-party group of MPs, led by the SNP, has warned Mr Cameron against holding the EU referendum in June, arguing it will be too close to elections in Scotland, Wales, Northern Ireland, London and local authorities.
What David Cameron said about the deal
The prime minister said “more work” needed to be done to “nail down” details but added: “We said we needed to deliver in four key areas, this document shows real progress on that front.”
He said the proposals were some “something worth fighting for”, and were good enough that he would back Britain joining the EU under these terms, if it was not already a member.
He said Britain could have the “best of both worlds” by giving it access to the single market and a voice around the top EU table, while retaining its status as a “proud independent country not part of a superstate”.
He said ministers would be free to campaign for either side in a personal capacity, but the government would “not be taking some sort of neutral position”.
“If we get this deal in February or in March or later and if the cabinet agrees to this deal the government’s position will be to campaign for Britain to stay in a reformed European Union.”
Asked by the BBC’s Laura Kuenssberg if he could guarantee the reforms would cut immigration and had not been watered down, he said: “I can say, hand on heart, I have delivered the commitments made in my manifesto.”
What those who want the UK to leave the EU say
Richard Tice, co-founder of Leave.Eu, accused Mr Cameron of “trying to deceive the British people by saying that there’s substantial change – there is nothing except a restatement of the existing status quo”.
The Vote Leave campaign said Mr Cameron had broken a key Conservative manifesto pledge to insist on a four year ban on in-work benefits, saying that the ban was now conditional.
Former defence secretary Liam Fox said the proposals did not “come close” to the changes voters had been promised.
UKIP leader Nigel Farage said Mr Cameron’s deal was “pathetic” and “hardly worth the wait”.
Donald Tusk’s reaction
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”.