More than 100 Tories have already confirmed they will reject Mr Cameron’s agreement with other EU leaders and campaign for Britain to leave the EU at the upcoming in/out referendum on June 23.
Yesterday, Mr Johnson ended weeks of speculation by announcing he will also join the Brexit bid, despite a last-ditch plea by the Prime Minister.
And Mr Cameron suggested he had taken the Mayor of London’s public snub personally, as he made a thinly-veiled attack on his Tory rival.
Delivering a statement to the House of Commons on the conclusions of last week’s marathon European Council, the Prime Minister highlighted suggestions Mr Johnson is backing Brexit as a means of boosting his chances of succeeding Mr Cameron in Downing Street.
Ending his statement to MPs, the Prime Minister said: “I am not standing for re-election.
“I have no other agenda than what is best for our country. I am standing here today telling you what I think.
“My responsibility as Prime Minister is to speak plainly about what I believe is right for our country. And that is what I will do every day for the next four months.”
Mr Cameron also mocked the idea – previously put forward by Mr Johnson – that Britons’ should vote to Leave in June in order to secure a second renegotiation deal from the EU on better terms.
In a direct attack on the Mayor of London, the Prime Minister said: “This is a vital decision for the future of our country. And we should also be clear that it is a final decision.
“An idea has been put forward that if the country votes to leave we could have a second renegotiation and perhaps another referendum.
“I won’t dwell on the irony that some people who want to vote to leave – apparently want to use a leave vote to remain.
He added: “Having a second renegotiation followed by a second referendum is not on the ballot paper.
“And for a Prime Minister to ignore the express will of the British people to leave the EU would not just be wrong, it would be undemocratic.
“On the diplomacy, the idea that other European countries would be ready to start a second negotiation is for the birds.”
But Mr Johnson hit back at the Prime Minister’s dig, later standing up in the Commons to launch an attack on his renegotiation deal.
To both cheers and jeers from MPs – as well as a shout of “tuck your shirt in” – the Uxbridge and South Ruislip MP said: “Can I ask the Prime Minister to explain to the House and to the country in exactly what way this deal returns sovereignty over any field of law-making to these Houses of Parliament?”
Mr Cameron replied: “This deal brings back some welfare powers, it brings back some immigration powers, it brings back some bailout powers, but more than that, because it carves us forever out of ever closer union, it means that the ratchet of the European court taking power away from this country cannot happen in future.
“To those who worry – and people do worry – that somehow, if we vote to remain in, the consequence could be more action in Brussels to try and change the arrangements we have, we have a lock in this House of Commons.
“No power can be passed from Britain to Brussels without a referendum of the British people.
“We have a better deal. We have a special status. We have a chance to make sure we build on what we have, we protect our people, we enhance our prosperity and that”s the choice we should make.”