David Cameron has issued a stark warning that leaving the EU will be a "leap in the dark" as he fired the starting gun in the referendum campaign which will determine Britain's European future.
The Prime Minister returned overnight from his marathon negotiations in Brussels to brief a rare Saturday meeting of the Cabinet on his reform package and to confirm his promised in/out referendum will be held on June 23.
The meeting was the signal for five Cabinet ministers headed by Justice Secretary Michael Gove - freed from the shackles of collective responsibility - to declare they would be campaigning for an "out" vote.
The group - plus Employment Minister Priti Patel who is not a full Cabinet member but attends meetings - immediately headed out from Downing Street to the Vote Leave campaign headquarters where they posed for photos with a "Let's take back control" poster.
In No 10, there was relief there were no last minute surprises among the list of "outers" - which also included Work and Pensions Secretary Iain Duncan Smith, the Leader of the Commons Chris Grayling, Culture Secretary John Whittingdale and Northern Ireland Secretary Theresa Villiers.
During a meeting which lasted more than two hours - with every minister present being given the opportunity to speak - potential waverers such as Business Secretary Sajid Javid, Attorney General Jeremy Wright and deputy Conservative chairman Rob Halfon confirmed they would be in the "in" camp.
A No 10 spokesman said the discussion had been conducted in "a good spirit and a dignified manner" with "measured, thought through interventions" from ministers reflecting the way they had each carefully weighed up the choice they were facing.
That left "out" supporters looking to Boris Johnson - who has assiduously kept his options open - as the possible high-profile champion that their campaign clearly craves.
There was speculation that the London mayor could use his weekly column in The Daily Telegraph finally to come off the fence and declare his hand.
Speaking on the steps of No 10, Mr Cameron said that the "special status" that he had secured for the UK meant the country could enjoy the benefits of the EU in terms of trade and security without having to sign up to a European superstate.
He said Britain inside the EU would be "safer, stronger and better off" while leaving would threaten the country's "economic and national security" creating deep uncertainty for the future.
"Those who want to leave Europe cannot tell you if British businesses would be able to access Europe's free trade single market, or if working people's jobs are safe, or how much prices would rise. All they're offering is a risk at a time of uncertainty - a leap in the dark," he said.
Mr Gove, one of the Prime Minister's closest political allies, said it was "the most difficult decision of my political life" to disagree with him, but that he had to be true to his principles.
"My instinct is to support him through good times and bad. But I cannot duck the choice which the Prime Minister has given every one of us," he said.
"I believe our country would be freer, fairer and better off outside the EU. And if, at this moment of decision, I didn't say what I believe I would not be true to my convictions or my country."
Labour leader Jeremy Corbyn, while confirming the Opposition would campaign for an "in" vote, said the renegotiation was a "missed opportunity".
"We will be campaigning to keep Britain in Europe in the coming referendum, regardless of David Cameron's tinkering, because it brings investment, jobs and protection for British workers and consumers," he said.
Ukip leader Nigel Farage dismissed the "truly pathetic deal" and urged voters to seize the "golden opportunity" to show that Britain would be better off being fully independent of Brussels.