David Cameron will begin his drive to sell his EU reform plan when he sets out details of the deal hammered out with Brussels to Parliament.
The Prime Minister's statement in the Commons will be a first chance for him to gauge reaction to the draft plan - drawn up with European Council president Donald Tusk - among Conservative MPs.
Mr Cameron is well aware he would never be able to satisfy hardline Eurosceptics set on leaving the EU – who immediately condemned it - but he will hope that he has done enough to convince middle ground Tories to swing behind the deal.
He received a major boost on Tuesday when Home Secretary Theresa May signalled she could be prepared to support the proposals, saying that they provided "a basis for a deal".
Mrs May was thought to be the most senior Cabinet minister considering opposing the plan and had been tipped as a possible leader of the "out" campaign in the forthcoming referendum on Britain's EU membership – now expected to be held in June.
However, in a statement, she said it was "encouraging" that key UK concerns about the "abuse" of EU free movement rules and the use of European law to block the deportation of foreign criminals were being addressed.
Disappointed out campaigners may now look to London mayor Boris Johnson – who has yet to declare which side he will support – to spearhead the drive for Britain to leave.
Mr Cameron said that there was still a lot of work to do to finalise the agreement ahead of a crunch meeting of EU leaders in Brussels on February 18 and 19.
However, he made clear that he believed that the fundamental elements were in place for a deal which he would recommend voters accept when the referendum is declared.
The Prime Minister still has to win the support of all the other 27 leaders when they gather later this month, but Foreign Secretary Philip Hammond indicated they were not expecting a "significant negative reaction" to the plan.
The deal includes an "emergency brake" on in-work benefits for EU migrants, protections for non-eurozone states and a legally-binding assurance that the UK is not expected to pursue integration through "ever-closer union".
Mr Cameron said it offered "the best of both worlds", giving Britain continued 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.
However it was roundly condemned by Eurosceptics with Conservative former defence secretary Liam Fox saying the proposals did not "come close" to the changes voters while Ukip leader Nigel Farage said it was "truly pathetic".
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 Prime Minister 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.
In a phone call on Tuesday, US president Barack Obama told Mr Cameron the United States supports the UK remaining in the EU.