David Cameron has been warned his proposed deal to curb benefits for European Union migrants could lead to a surge in the number coming to the UK before the "emergency brake" mechanism is applied.
The Prime Minister's EU referendum strategy has come under fire from his own grassroots activists and he has also faced reported unrest within his top team as he prepares for a crunch summit with his counterparts in Brussels on February 18.
In a further sign of the divisions on his own side, former leadership contender David Davis warned that coverage of the proposed curbs on benefits would act as an incentive to workers to head to the UK over the coming months before a deal can be implemented.
Former Europe minister Mr Davis warned: "The so-called emergency brake that the Prime Minister is attempting to negotiate with Brussels is very likely to increase the number of people immigrating into the UK in the coming year."
He said since the proposed deal was set out "Eastern European newspapers have carried numerous stories about in-work benefits and the plans to terminate them for the first four years after a migrants arrival in the UK" while Brussels has suggested that the scheme could take 12 months to implement.
"Under such circumstances the incentive for anybody wishing to come to live in the UK will be to come as quickly as possible to beat the deadline when any such restrictions come into effect.
"Accordingly we are likely to see a surge in migrants in the next 12 months."
Former cabinet minister Sir Eric Pickles, a Eurosceptic who now looks likely to support the Prime Minister's deal, said: "What we need to be clear is, we're not trying to prevent people moving inside the European Union."
Instead the reforms were aimed at preventing people gaining "something for nothing" by gaining benefits before contributing to the UK.
The Tory MP for Brentwood and Ongar said action would be needed to prevent a surge of migrants hoping to beat the emergency brake: "Clearly as part of the negotiations we have to ensure that that doesn't happen."
He added: "We've got to ensure that there isn't a new influx."
Sir Eric played down the prospect of the European Parliament rejecting the reform proposals, claiming it would be an "appalling misuse of trust that would fundamentally undermine" the EU.
He said his own position was not the result of "any enthusiasm for Europe" but "the lack of any decent idea that we'd be better off outside".
In a boost to Mr Cameron, one of Margaret Thatcher's closest advisers suggested the former prime minister would have backed his proposed deal for a new relationship with Brussels.
Lord Powell of Bayswater, Mrs Thatcher's former private secretary and foreign affairs adviser, used a Sunday Times article to say that although the so-called Iron Lady might have "raged more mightily" at Brussels than Mr Cameron, "she would have gone along with what is on offer, indeed negotiated something similar herself".
But in a sign of the unease in Tory ranks, local party chiefs complained about Mr Cameron's direction to MPs not to worry about their views on Brussels.
The Prime Minister was accused of showing "disrespect" to party activists in a letter signed by representatives from more than 40 local Conservative associations.
The letter by local party chiefs to The Sunday Telegraph came in response to Mr Cameron's plea in the Commons for his MPs to ignore the views of their constituency associations when deciding on the EU question.
"Don't take a view because of what your constituency association might say, or you're worried about a boundary review, or you think it might be advantageous this way or that way," he told MPs on Wednesday.
In a letter to The Sunday Telegraph, the Conservative associations say Mr Cameron has "undermined" the goodwill that existed among loyal members and warned him: "No prime minister has a divine right to rule."
The missive is signed by 44 chairmen, executives, former chairmen and Tory activists, representing 43 local party associations across England, Wales, Scotland and Northern Ireland.
A Downing Street spokesman told the newspaper: "The PM has the greatest of respect for Conservative associations across the country. He was simply making the point that everyone should ultimately vote with their conscience."
Meanwhile, speculation continued to surround the likely positions of Cabinet ministers when the referendum campaign finally gets under way.
Reports suggested that employment minister Priti Patel is likely to back a Brexit, while The Sunday Telegraph claimed Mr Cameron had personally appealed to Justice Secretary Michael Gove not to join the campaign for the UK to sever ties with Brussels.
News Corporation boss Rupert Murdoch tweeted: "Cameron's EU non-deal shocks UK opinion. Now he's rumoured to be begging cabinet colleagues to put aside beliefs."
The Sunday Times reported that Work and Pensions Secretary Iain Duncan Smith, a prominent Eurosceptic expected to campaign for Brexit, had told friends Mr Cameron's deal was a "load of rubbish" which would be challenged in the European courts.
Meanwhile Boris Johnson's position remained in doubt, with the Mail on Sunday reporting that he told fellow Tory MP Bernard Jenkin he had "never been an 'outer'".
Mr Jenkin said: "Boris was quite open and frank about it - it was no secret conversation. He is genuinely torn but I hope he will change his mind because he is one of the few who understands that if we stay in, Britain will be in a weaker position than ever before."