Lib Dem leader Tim Farron accused Mr Cameron of acting in the interest of the party and not the country by allowing ministers to campaign against European Union membership.
The BBC reported that Cameron will announce sometime on Tuesday he has given his ministers a green light for campaigning for a Brexit if they want to - something he is staunchly against - after he has finished trying to "seal a better deal" for Britain within the EU.
I have supported the Prime Minister's efforts to secure a new deal from the European Union, but it has become clear that despite the Greek crisis, the refugee crisis, border control chaos, and economic stagnation the European Union still doesn't see the need for fundamental reform'.
The Prime Minister is due to meet his fellow European Union leaders again in February as he moves to finalise changes in four area.
Had he insisted that his ministers had to agree with whatever stance he took, he faced the prospect of resignations or sackings.
Details of the policy are expected to be set out by Mr Cameron when he reports to the Commons this afternoon on December's summit, at which hopes of securing an early agreement were dashed.
GETTYTory MP John Redwood told Mr Cameron he'll have to reign after a Brexit vote
Mr Johnson said: "The Prime Minister has got a negotiation to conduct and I am obviously supporting him".
The Mayor claimed "glutinous harmony" prevailed within the Tory party - including Mr Grayling and Ms Villiers - and MPs were backing their leader.
Mr Johnson denied that Conservative MPs were fighting over Europe: "We're not".
"The British people deserve to hear where their elected representatives stand on this vitally important issue". There has been speculation that some euro-sceptic ministers would have resigned as ministers if he had forced them to campaign for membership.
A Downing Street spokesman said ministers will be told their views will be "accommodated" but only after Mr Cameron finishes his renegotiation of Britain's relationship with Brussels. The question of whether collective responsibility would apply after the renegotiation is concluded would be answered "at a later stage", she said.
