David Cameron came under pressure to say he will allow Government ministers to campaign for a British exit from the European Union, with Boris Johnson saying giving senior Tories a free vote would be "safer and more harmonious".
The intervention by the mayor of London, a member of the Prime Minister's political Cabinet, came after Mr Cameron insisted he would not announce whether ministerial colleagues will be bound by collective responsibility until nearer the time of the public vote.
The legislation for the the in/out vote on membership of the European Union faced its first test in the Commons, where Foreign Secretary Philip Hammond came under pressure from Eurosceptics over the role the Government would play in the run-up to the referendum.
Mr Johnson's comments fuelled the row over whether the Prime Minister would sack ministers who campaign to leave the EU against his advice.
Asked if ministers should be allowed to campaign for "Brexit" and keep their posts, Mr Johnson said: "I don't see why not myself."
He added: "I think, probably, it would be safer and more harmonious just to say 'OK, you make your minds up'. I think, on something like this, do you really need to bind everybody in?"
He said that if Mr Cameron got the deal he wanted in Europe, "the overwhelming majority of his colleagues - on both the front and the back benches - will support him".
Mr Cameron has vowed to renegotiate the UK's relationship with Brussels before an in/out vote by the end of 2017.
Mr Johnson left the door open to campaigning for Brexit if the Prime Minister did not achieve an acceptable outcome in his talks with EU leaders, saying: "Let's see where we get to."
He suggested that, unless there was treaty change to make amendments to benefits rules for European migrants demanded by Mr Cameron, the Prime Minister would be forced to recommend a No vote in the referendum.
Mr Johnson said it was clear that "unless we can get treaty change to prevent migrant workers from being able to access benefits before they have been here for four years, there will will be no alternative but to recommend a No".
At the weekly Cabinet meeting Mr Cameron "reiterated his position on the approach to renegotiation and the referendum", Downing Street said.
The Prime Minister's spokeswoman said: "The position is very clear. All the Government are behind engaging with our European partners to renegotiate the UK's relationship.
"The decision on the approach during the referendum is for further down the road."
Asked about Mr Johnson's comments, she said: "The Prime Minister is clear that he thinks the approach during the referendum is an issue for further down the road, we need to take this step by step, the focus very much now is on renegotiating our relationship and addressing the concerns of the British people."
Eurosceptics, including former Cabinet minister Owen Paterson, have claimed the Government could try to "fix" the referendum by scrapping the usual purdah restrictions preventing the publication of promotional material by Whitehall before a poll.
Number 10 defended the position, saying Mr Cameron had been clear that "the Government is not going to be neutral on this".
Challenged about the move in the Commons, the Foreign Secretary said there were "operational and political" reasons for lifting the restrictions but insisted that the Government did not plan to "spend large sums of public money" campaigning.
If the restrictions were not lifted they "would stop the Government from 'publishing' material which deals with 'any issue raised by' the referendum question", Mr Hammond told MPs.
"In the context of this referendum that is unworkable and inappropriate", he said, adding that Government ministers would want to campaign without "being constrained by fears" over whether writing a comment on Twitter would breach the law.
Tory Eurosceptic Peter Bone questioned him, stating that "a lot of people are concerned that the Government will use the apparatus of state to push a case rather than letting the two sides have equal and fair access".
But Mr Hammond insisted that the Government would exercise "proper restraint to ensure a balanced debate" and insisted there was no intention of "spending public money to deliver doorstep mail shots in the last four weeks of the campaign".
Mr Paterson claimed the Government plans would allow taxpayers' money to be spent on a "deluge of propaganda" during the referendum campaign which could leave the public with a sense the vote is "rigged".