Boris Johnson has said ministers should be allowed to campaign for Britain to leave the European Union, amid an internal-Conservative Party row over the referendum.
On Monday, David Cameron appeared to perform a swift U-turn on whether he would allow members of his government to keep their jobs and join the 'out' campaign.
Initially the prime minister had suggested ministers would have to quit if they wanted to argue in favour of Britain leaving the EU. However yesterday afternoon Cameron insisted he had not made up his mind and blamed journalists for misunderstanding what he had said.
Today, Boris heaped pressure on Cameron when he said it would be "safer and more harmonious" if cabinet ministers were given the freedom to campaign on whichever side they wanted.
Cameron's official spokesperson said of Boris' comments: "The prime minister is clear he thinks the approach during the the referendum is an issue for further down the road. We need to take this step by step. Of course people are going to express views. There are going to be a lot of views during the re negotiation."
Foreign secretary Philip Hammond insisted today that Cameron had been "consistent" in his position and had been "misinterpreted" by reporters.
He told MPs it was "simply hypothetical to talk about who will be allowed to do what" in the campaign as the outcome of the renegotiation was not yet known.
Boris told LBC radio: "I think in 1975 from memory I think cabinet ministers were allowed to campaign against staying in and to keep their positions. I don’t see why not myself.
"Just thinking about it out loud on the spur of the moment let me think: I think probably it would be safer and more harmonious just to say 'OK you make your minds up'.
"I think probably on something like this, do you really need to bind everybody in? There will be different views, get it over.
Boris added: "The prime minister will be able to make a recommendation. It’s almost certain if he gets the deal that he wants the overwhelming majority of his colleagues on both the front and backbenches will support him."
The London mayor was appointed to Cameron's political cabinet following the general election.
MPs will have their first opportunity to debate the government's EU referendum Bill in the Commons today. The legislation should have no trouble passing as Labour ditched its pre-election opposition.
However shadow foreign secretary Hillary Benn has said Labour will refuse to share a platform with Conservatives who campaign to stay in the EU. "We won’t be sharing a platform with David Cameron," Benn told the Daily Mirror. "He can’t even tell us whether he is in favour of staying in the EU or not."
Hammond has been identified as one potential 'out' campaigner, as have Iain Duncan Smith, Sajid Javid, Michael Gove and Michael Fallon.
Asked if he would join the 'out' campaign, Hammond told the Commons that like the prime minister he "rules nothing out".
Over the weekend 50 Tory MPs formed a new 'Conservatives for Britain' group designed to put pressure on Cameron to secure a radical new relationship with the EU.