David Cameron will begin his drive to sell his EU reform plan amid stinging criticism, when he sets out details of the draft 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 plan - drawn up with European Council president Donald Tusk - among Conservative MPs.
Some cabinet ministers are threatening to defy Cameron by publicly speaking out against his deal, amid claims it will fail to cut migration, The Telegraph reported.
Cameron is trying to sell the draft deal to MPs
The deal has been called "watered-down", "pathetic" and "insubstantial" according to the paper, because it would give migrants from the EU increasing access to benefits in the UK rather than the ban Cameron had asked for.
Senior Conservatives have reportedly slammed Cameron for "wasting a once in a generation opportunity” to renegotiate Britain’s membership of the EU, according to The Sun.
Ukip deputy leader Paul Nuttall said: "The theatrics and drama of David Cameron's sham renegotiation continues and he is playing us for fools. When Cameron confirms a 'deal' with Brussels, it looks like it'll be nothing more than tinkering round the edges of our relationship with the European Union."
But Labour MP and Former Shadow business Secretary Chuka Umunna praised the deal, saying it was one "the left can support".
"Thankfully, the Prime Minister has heeded the warnings the labour movement made about using this renegotiation to weaken people's rights at work," he wrote in the New Statesman. "Nothing on these lines has appeared in the text of the draft agreement circulated to EU leaders today. The changes – an opt-out from ‘ever closer union’, more competitiveness in the European economy, protections for countries that do not use the euro, and action on benefits paid to EU migrants – do not endanger any of the hard-won rights and benefits that the EU has given working men and women in Britain.
"While the reforms are hardly earth-shattering, they are substantial and will make our membership of the EU that bit more attractive."
Ummuna: 'Left can support' the deal
The Press Association reported that Cameron is well aware he would never be able to satisfy hardline Eurosceptics set on leaving the EU – who immediately condemned the plan - but he will hope that he has done enough to convince middle ground Tories to swing behind the deal.
Cameron 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".
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.
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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.
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.
Cameron with European Commission President Jean-Claude Juncker
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".
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 wanted, while Ukip leader Nigel Farage said it was "truly pathetic".
There was concern that 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 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 Cameron the United States supports the UK remaining in the EU.