David Cameron is to set out his EU reform plan to his senior ministerial team as he vowed to campaign "heart and soul" for the deal finally agreed in marathon talks in Brussels.
The meeting of the Cabinet - the first to be held on a Saturday since the Falklands War - will effectively fire the starting gun in the campaign for the Prime Minister's long-promised the referendum now expected to be held on June 23.
It will mark the moment when ministers such as Iain Duncan Smith and Chris Grayling opposed to Britain's membership of the EU can finally go public and say they will be campaigning to leave.
Mr Cameron said he was "disappointed but not surprised" they now look set to include his long-time political ally Justice Secretary Michael Gove who, after much soul-searching is reported to have joined the "out" camp.
Speaking at a late night Friday news conference - following talks which began on Thursday afternoon and went on through much of the night - an emotional Mr Cameron said the package he had agreed had secured Britain's "special status" in the EU.
"Britain will be permanently out of 'ever closer union', never part of a European superstate," he said.
"I believe that this is enough for me to recommend that the United Kingdom remains in the European Union, having the best of both worlds.
"This is why I will be campaigning with all my heart and soul to persuade the British people to remain in the reformed European Union that we have secured today."
The changes fulfilled the reform objectives in the Conservatives' general election manifesto and were "legally binding" and irreversible without the agreement of all EU member states including the UK, he said.
He added: "Whatever the British people decide, I will make work to the best of my abilities. But let me tell you what I believe: I do not love Brussels; I love Britain."
The agreement was swiftly dismissed by Ukip leader Nigel Farage as a "truly pathetic deal".
"We are good enough to be an independent, self-governing nation outside of the EU. This is our golden opportunity," he said.
The new deal will allow host nations to cut migrants' child benefit payments for children living overseas to the rate paid in their home countries - usually far lower than those received by UK parents.
A compromise will allow existing claimants to carry on receiving child benefit in full for offspring living overseas until 2020, and all member states will then be able to pay them at the rate of their home country.
It falls well short of the outright ban on sending child benefit abroad initially demanded by Mr Cameron, and marks a compromise with eastern European states who had insisted that existing claimants should continue to receive the full payment until their sons and daughters reach adulthood.
An "emergency brake" on in-work welfare payments for future migrant workers will be made available for seven years - with no option for extensions - in cases where member states are facing excessive strain from new arrivals.
The seven year period is shorter than the 13 years put forward by Mr Cameron in negotiations, but considerably longer than eastern European nations had argued for.
The new deal also says EU treaties will be amended to state explicitly that references to the requirement to seek ever-closer union "do not apply to the United Kingdom".
It came after behind-the-scenes talks which stretched through Thursday night and most of Friday, as Mr Cameron and European Council president Donald Tusk struggled to keep Britain's renegotiation on track.
The 28 leaders had initially been due to gather early in the morning for an "English breakfast" meeting to approve a package of reforms to the UK's membership, but breakfast became brunch, lunch, high tea and then dinner as opponents of the deal dug in their heels.
The delays forced Mr Cameron to scrap plans to summon ministers for a Cabinet meeting on Friday evening.
European Council president Donald Tusk said the settlement "addresses all of PM David Cameron's concerns without compromising our fundamental values".
Mr Tusk, who oversaw months of negotiations leading to the deal, added: "We have achieved a legally binding and irreversible deal decided on by all 28 leaders, strengthening Britain's special status in the EU.
"We didn't walk away from the negotiating table. We were willing to sacrifice part of our interests for the common good, to show our unity."
But Belgian PM Charles Michel suggested the deal amounted to a climbdown by Mr Cameron on migrants' benefits, saying: "We are very, very far from the initial British position that wanted to create a discrimination."
CBI director-general Carolyn Fairbairn, said the reform package looks to be "a major step forward" on the journey to a more competitive and prosperous future for Britain within the EU.
"These reforms protect the UK's place and influence inside this important market and a renewed focus on EU competitiveness will help British firms succeed in creating jobs and economic growth at home in the years ahead," said Ms Fairbairn.