David Cameron takes his battle to keep Britain in the EU to MPs on Monday 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. The prime minister will publish a white paper on his EU reform plans before making a statement to MPs on last week’s European council. In a sign of a near collapse in relations between the pair, the London mayor texted Cameron at 4.40pm to tell him of his plans minutes before explaining his “agonisingly difficult” decision on his north London doorstep. 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 éclat”, marks a severe blow to the remain campaign and will transform the fortunes of the leave…