Boris Johnson argued that Britons should "think of the future" and remain in the EU in a secret article he wrote just two days before revealing he would be campaigning for Brexit.
The Foreign Secretary argued that despite the "defects and disappointments" of the UK's EU deal, it would be a "boon for the world and for Europe" if Britain stayed in the bloc.
Johnson, who is now seen as a backer of a "hard Brexit", this week described the EU single market as an "increasingly useless" concept.
But in the pro-EU article, revealed in a new book and published in The Sunday Times (£), he supported membership of the free trade zone.
In the article, he wrote: "This is a market on our doorstep, ready for further exploitation by British firms.
"The membership fee seems rather small for all that access.
"Why are we so determined to turn our back on it?"
He appealed to Leave voters to consider the impact Brexit could have on the lives of future generations.
Think of the desire of your children and your grandchildren to live and work in other European countries; to sell things there, to make friends and perhaps to find partners there. Ask yourself: despite all the defects and disappointments of this exercise - do you really, truly, definitely want Britain to pull out of the EU?
In the article, Johnson warned that Brexit would cause an "economic shock" and could lead to the "break up" of the UK.
Since the vote to leave the EU the pound has fallen to historic lows, losing around 18% of its value against the US dollar.
And on Saturday, Scottish First Minister Nicola Sturgeon has put in place plans for a second independence referendum if the UK leaves the single market.
Johnson concluded the article by describing the EU deal as a "bit of a dud" but added: "It contains the germ of something really good. I am going to muffle my disappointment and back the prime minister."
Sources close to Johnson said he wrote the article to try and articulate his mind to the Remain argument but dismissed it out of hand as soon as he finished.
In the book, All Out War, by the newspaper's political editor Tim Shipman, it is claimed Johnson "wanted to punch" his Leave ally Michael Gove after he ran as Tory leader effectively sabotaging his own hopes of becoming Prime Minister.
Boris Johnson's pro-EU article in full:
OK OK, I admit it. If you gave him a truth drug, or hypnotised him, I don't think even the prime minister would really deny it.