Boris Johnson has said that ministers should be allowed to campaign freely in the upcoming EU referendum.
Government ministers should have free reign to campaign on their conscience, he said, adding it would be "safer and more harmonious" for David Cameron to let them do so.
As MPs debated in the House of Commons over the government's EU referendum bill, Johnson made the move to put increased pressure on the Prime Minister.
Cameron has come under fire after his perceived u-turn on the matter, after claiming he was misinterpreted in saying that ministers would lose their jobs if they campaign for an exit against Cameron's wishes.
The EU Referendum Bill is expected to pass its first legislative stage in Parliament this evening, authorising a vote before the end of 2017.
Speaking on LBC radio, the London mayor and MP for Uxbridge said that "I don't see why not" when asked if ministers should be allowed to campaign for Brexit.
He said it was for the Prime Minister to decide, but that such freedom "seemed to work" in 1975, when the UK had its last EU referendum.