Senior Conservatives have rejected David Cameron's claim that the EU reform deal he secured in Brussels will cut immigration, as new figures showed 172,000 migrants arrived from Europe last year.
Work and Pensions Secretary Iain Duncan Smith and employment minister Priti Patel both said that the only way to stem numbers of incomers was for Britain to regain control of its own borders. Ms Patel said the Prime Minister's deal - which envisages a four-year emergency brake on migrants' benefits - would "do nothing" to reduce arrivals.
Her comments came as Press Association research found that Mr Cameron has yet to persuade half of his MPs to offer public support for his battle to keep Britain in the EU. As of Thursday afternoon, some 157 of his 329 Commons colleagues had announced they will vote to remain, against 128 advocating Brexit and 45 undeclared.
Conservative MPs clashed over Europe in the House of Commons, where former defence minister Sir Gerald Howarth launched an attack on the Prime Minister's claim that continued EU membership was good for Britain's security.
By suggesting the EU has a role in the defence of Europe, the Aldershot MP said Mr Cameron was "playing into the hands" of supporters of an EU army and risked undermining Nato.
But Foreign Secretary Philip Hammond dismissed the charge, telling MPs: "We've always been very clear that any role of the European Union in relation to our defence must be complementary to and in no way undermine the role of Nato."
Mr Hammond warned that goodwill towards Britain from its EU partners would "evaporate in an instant" if the UK votes to leave on June 23.
"To those who say they will have to negotiate a sweetheart trade deal with the UK outside the EU, I say this: There will be no desire at all among the political elites of the remaining 27 member states to help an exiting Britain show that it can prosper outside the EU," Mr Hammond told MPs.
"Quite the contrary: They will interpret a leave decision as two fingers from the UK and we can expect precisely the same in return."
In a thinly veiled swipe at London Mayor Boris Johnson, who has floated the idea of a leave vote being followed by a second referendum under better terms, Mr Hammond said: "No individual, it doesn't matter how charismatic or how prominent, has the right or the power to redefine unilaterally the meaning of the question on the ballot paper."
As a number of Tory MPs tried to pick holes in Mr Cameron's reform package, shadow foreign secretary Hilary Benn told the Commons that, while Labour was divided and Conservatives united over Europe 40 years ago: "There has been a complete reversal of roles ... History is repeating itself in mirror image."
Meanwhile, Mr Cameron told workers at defence giant BAE Systems in Warton, Lancashire, that "the safe choice, the right choice, the pro-jobs choice, the pro-investment choice", was to stay in a reformed EU.
The PM questioned how many of the UK's three million jobs linked to trade with Europe would "truly be safe" if Britain quit the EU. And he asked. "In a dangerous and uncertain world, why take the leap in the dark? That is at the heart of the case I am making."
Home Secretary Theresa May said that figures released by the Office for National Statistics (ONS) showed net migration remains too high. But she insisted Mr Cameron's reform package will "reduce the pull factor of our welfare system" on migrants.
However Ms Patel, who is campaigning for Brexit, said: "The proposed deal will do nothing to reduce the level of immigration from the EU. The only way to take back control is to vote leave."
Ukip said the numbers arriving in 2015 were enough to fill a city the size of Hull, while leader Nigel Farage pointed to ONS figures showing an 8% rise to 828,000 in the volume of National Insurance (NI) numbers issued to foreigners - 630,000 of them to EU nationals - over the course of the year.
The ONS stressed that NI numbers include short-term visitors, who are not included in net migration statistics, as well as migrants who had arrived in previous years but only got round to registering for NI in 2015. But Mr Farage insisted that they represented "a simple and clear reflection of the real numbers of people in this country, as without them you can neither legally work, nor claim benefits".
Mr Duncan Smith warned that Mr Cameron's emergency brake arrangement could even cause a short-term spike in immigration as would-be migrants seek to establish residency before its likely introduction in spring 2017.
And he warned failure to address immigration concerns could result in the rise of extreme right-wing parties.
"If you do not control your borders, my observation is that you get parties led by people like Marine Le Pen and others who feed off the back of this, and ordinary decent people feel life is out of control," he said.