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The Foreign Secretary wrote the unpublished Remain-backing article only two days before shocking David Cameron by revealing he would be campaigning for Brexit.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 per cent 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.
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.
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