London: David Cameron is taking his battle to keep Britain in the EU to MPs after hitting the most serious political obstacle yet when Boris Johnson announced on Sunday that “after a huge amount of heartache” he is to back the leave campaign.

On Monday, the prime minister was to publish a white paper on his EU reform plans before making a statement to MPs on last week’s summit after suffering the setback of seeing one of his party’s most popular figures defy him.

The ever loyal Michael Fallon launched a cabinet fightback against the Brexit campaign after a weekend in which it dominated the headlines when six ministers formally endorsed the Vote Leave group and Johnson declared his hand. The defence secretary challenged claims by Iain Duncan Smith , one of the ministers, that the UK will be more exposed to Paris-style terror attacks if it stays in the EU. Fallon said that the UK’s defence and security rests on Nato but the EU adds to the UK’s security.

The defence secretary told the Today programme on BBC Radio 4: “I don’t know of any member of Nato that wants us to leave the EU because the EU can do things Nato can’t. For example we were able to persuade the rest of Europe to apply sanctions against Russia after the annexation of Crimea and its interference in Ukraine. It was the EU that applied sanctions against Iran legally to get them to abandon the civil nuclear programme.”

He also pointed out the benefits for fighting crime and terrorism: “It is through the EU you exchange criminal records and passenger information and work together on counter-terrorism. So the EU is not wholly our security, of course that is Nato. But it complements the security we have in the west.”

But Lord Lawson of Blaby, the former chancellor who chairs the Vote Leave campaign group, said that leaving the EU would mark a declaration of independence by the UK. “It is about time we became a self-governing independent democracy. We have a far better future economically, politically and in every other way once we have achieved that.”

In a sign of a near collapse in relations between the prime minister and the London mayor over the referendum issue, Johnson had texted Cameron at 4.40pm on Sunday to tell him of his plans minutes before explaining on his north London doorstep about his “agonisingly difficult” decision to break with the government.

Johnson told a scrum of reporters and cameramen outside his Islington home: “I will be advocating Vote Leave — or whatever the team is called, I understand there are a lot of them — because I want a better deal for the people of this country, to save them money and to take control. That is really what this is all about.”

The intervention by the London mayor, which he had promised last week would amount to a “deafening eclat”, marks a severe blow to the campaign to stay in the EU and will transform the fortunes of the leave campaign. It is also a personal humiliation for the prime minister who had urged Johnson hours earlier to avoid “linking arms” with Nigel Farage and George Galloway in backing a British exit. In an email on Saturday morning Johnson had warned Cameron that he was likely to back the leave side but he received no reply.

Johnson, who has faced criticism that he made his decision in a way calculated to guarantee his best chances in the next Tory leadership contest, tried to make clear that he was acting out of conviction. In his statement outside his home , the London mayor praised the prime minister for doing “fantastically well” in his EU negotiations.

“I think everybody should pay tribute to David Cameron for what he pulled off in a very short space of time,” Johnson said. But he then added that the prime minister had failed in his main goal to reform the EU. “I don’t think anybody could realistically claim that this is fundamental reform of the EU or of Britain’s relationships with the EU.”

Johnson added: “I would like to see a new relationship based more on trade, on co-operation, with much less of this supranational element. So that is where I’m coming from and that is why I have decided, after a huge amount of heartache, because the last thing I wanted was to go against David Cameron or the government, I don’t think there is anything else I can do.”

Downing Street has been irritated by Johnson who has, in their eyes, dithered in making up his mind over the last year. Over the summer he flirted with the idea — promoted by the Vote Leave campaign director, Dominic Cummings — to hold two referendums to reassure undecided voters that the initial one would not mark a definitive break.

The Vote Leave campaign was delirious at Sunday’s announcement. It will have a simple answer to criticisms that they are only supported by politicians on the fringes of the main parties. Vote Leave tweeted a new version of Johnson’s London mayoral campaign cartoon of him with the simple message: “Welcome aboard, @BorisJohnson ! #VoteLeave .”

Downing Street issued a low-key response. A No 10 spokesman said: “Our message to everyone is we want Britain to have the best of both worlds: all the advantages of the jobs and investment that come with being in the EU, without the downsides of being in the euro and open borders.”

But the Britain Stronger in Europe group showed its concern by wheeling out Lord Heseltine, the former Tory deputy prime minister who was Johnson’s immediate predecessor as MP for Henley during his first stint in parliament. Heseltine said: “Given that Boris has spent so long agonising over this decision, his decision is illogical. If it takes you this long to make up your mind about something so fundamental and you still have questions, then surely the right option is to stay with what you know rather than risk our economy and security with a leap in the dark.”

In his statement Johnson, addressed claims that he was being an opportunist to boost his Tory leadership credentials. “The big battalions of the argument are unquestionably ranged against people like me. We are portrayed as crazy, cracked and all the rest of it,” Johnson said.

“I don’t mind. I happen to think that I am right. It is a very, very difficult case to make. I have thought an awful lot about it. I have thought about it for many years.”