David Cameron has warned that leaving the European Union “could hurt working people for years to come” as he put the case for staying in the EU to MPs.
He said the choice was between an “even greater Britain” by staying in, or a “leap into the dark” by exiting.
There were thinly veiled swipes at Boris Johnson, including the PM ruling out the idea of a second referendum.
More than 100 Conservative MPs want to leave the EU, including some ministers listening to his Commons statement.
In a two and a half hour statement to the House of Commons, Mr Cameron told MPs that, as a prime minister who was not going to seek re-election, he had “no other agenda than what is best for our country”.
That was seen as pointed reference to Mr Johnson, has been accused by some of putting personal political ambition ahead of principle in deciding to campaign for EU exit.
The Conservative MP has rejected that suggestion and insisted that he has long been sceptical of the benefits of UK membership and the UK has a “great future” outside it.
As he left the Commons on his bicycle after the EU statement, Mr Johnson replied “no” when asked if there was a civil war within the Conservative Party on the issue.
“It’s glutinous harmony”, he added.
Earlier he intervened during the PM’s statement to ask how the deal he negotiated would “in any way” return sovereignty to the UK.
The prime minister defended the deal he negotiated with the EU’s 27 other states, outlined in a new government document, telling MPs it would give the UK a “special status” within the EU and ensure it never became part of a European super-state.
James Landale, BBC deputy political editor
Abraham Lincoln said that “a house divided against itself cannot stand”. So how can the government stand when it is so utterly divided over Britain’s future in the European Union?
The short answer is that it has simply decided to suspend the usual rules demanding unity and loyalty.
Cabinet ministers have been given permission to ignore collective responsibility which requires them to support government policy.
That means that the five full members of the cabinet who are refusing to support the government’s position of backing the Remain campaign can stay in their jobs.
But, in a letter published last month, David Cameron set out some pretty tight constraints on what those ministers can do.
The UK, he said, would be “safer and stronger” as a result of a exemption from ever-closer union, limits to in-work benefits for EU migrants that he said could last up to 2028 and protection for countries outside the eurozone, telling MPs that the UK was “better off fighting from the inside”.
The prime minister dismissed talk of a second referendum on the terms of withdrawal if the British people voted to leave in four months’ time, saying this option was “not on the ballot paper”.
In the event of a vote to leave, he said Article 50 of existing EU treaties – the as-yet unused mechanism by which a country could leave the EU – would be triggered straight away and the process of separation would be difficult to reverse. If negotiations were not concluded within two years, he warned that many existing benefits of UK’s membership would lapse automatically.
He also challenged those backing EU exit to set out their alternative vision for the future of the country.
“I recognise there are disadvantages of being in the EU but I can look the British people in the eye and say this is what it is going to be like if we stay in,” he said. “The people who are advising us to leave have got to spell out what the consequences of leaving are.”
The statement was the first opportunity Tory MPs have had to question the PM since Friday’s agreement and publicly set out their position ahead of the poll in four months time.
A succession of Tory MPs questioned the substance of the PM’s agreement, suggesting it would do nothing to remedy the unfairness of the EU’s Common Agricultural Policy, reduce levels of EU immigration and address the power of European courts.
Jacob Rees-Mogg, one of 106 Conservative MPs – including five cabinet ministers – to so far publicly back EU exit, said the EU was a “failed” body and the UK should “make our own path”.
“For so much labour, he has achieved so little,” he said of the PM’s renegotiations. “Is the government’s policy basically ‘always keep a hold of nurse for fear of finding something worse?”
For Labour, Jeremy Corbyn said he was glad the “theatrical sideshow” of Mr Cameron’s negotiation was over and Labour could make a “real” argument for the benefits of EU membership while former SNP leader Alex Salmond urged him to make a “more positive case” for EU membership.
Asked about the apparent criticism of Mr Johnson, David Cameron’s spokesman said the prime minister – who will address a meeting of Tory MPs on Monday evening – did not have anyone in mind when he made the comments and “was speaking for himself’.
The government has published further details of the process leading up to the referendum . According to BBC research, 122 Tory MPs have so far publicly backed staying in the EU, 106 have come out against while 102 have yet to make their position clear.
The overwhelming majority of Labour MPs back continued EU membership as do the SNP, Plaid Cymru and the Liberal Democrats but the Democratic Unionists and UKIP are opposed.
Earlier on Monday, Defence Secretary Michael Fallon rejected claims that the UK’s membership exposed it to greater security risks, pointing out that the EU had taken the lead in confronting Russia over its annexation of Crimea and Iran over its nuclear programme.
Explaining his decision to back EU exit in the Daily Telegraph, Mr Johnson wrote: “There is only one way to get the change we need – and that is to vote to go; because all EU history shows that they only really listen to a population when it says no,” he wrote.