Boris Johnson said Britain remaining in the European Union would be a “boon for the world and for Europe” in a previously secret newspaper column.
The Foreign Secretary wrote the unpublished Remain-backing article only two days before shocking David Cameron by revealing he would be campaigning for Brexit.
Mr Johnson is now seen as a backer of a “hard Brexit”, this week insisting the UK can get a trade deal that is “of greater value” to the economy than access to the EU single market, which he described 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.
“This is a market on our doorstep, ready for further exploitation by British firms,” Mr Johnson wrote.
“The membership fee seems rather small for all that access.
“Why are we so determined to turn our back on it?”
Sources close to Mr Johnson said he wrote the article for the sole purpose of trying to articulate in his mind whether there was any merit in the Remain argument and dismissed it out of hand as soon as he finished.
He also warned that Brexit would cause an “economic shock” and could lead to the “break up” of the United Kingdom in the article published in All Out War, by the newspaper’s political editor Tim Shipman.
Since the vote to leave the EU the pound has fallen to historic lows, losing around 18% of its value against the US dollar, while Scottish First Minister Nicola Sturgeon has put in place plans for a second independence referendum if the UK leaves the single market.
Mr Johnson said that the deal Mr Cameron struck with Brussels after months of wrangling over Britain’s future relationship with the EU was “a bit of a dud”.
However he said elements of the accord, including removing the UK from ever closer union, would be “very powerful” once signed off in treaty.
Mr Johnson also 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 referendum an overwhelming majority of young people voted to Remain.
The book also claims Sir Lynton Crosby told Mr Johnson to support Brexit once Mr Cameron had ignored the election strategist’s advice to delay the referendum.
Among the other revelations, the Remain campaign’s digital specialist, Jim Messina, apparently described Mr Cameron’s pollster Andrew Cooper as “the worst I’ve ever worked with” for getting his forecasts about the vote drastically wrong.
And it said Mr Johnson “wanted to punch” Michael Gove after his Leave campaign ally ran in the subsequent Tory leadership race alone and in effect torpedoed the former London mayor’s candidacy and hopes of becoming PM.
Meanwhile, the Treasury described as “totally untrue” a Mail on Sunday claim that Chancellor Philip Hammond was set to quit the Government over its apparent shift towards leaving the single market.