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The Foreign Secretary wrote the unpublished Remain-backing article only two days before shocking David Cameron by revealing he would be campaigning for Brexit.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.
What experts have said about Brexit
What experts have said about Brexit
1/11 Chancellor of the Exchequer Philip Hammond
The Chancellor claims London can still be a world financial hub despite Brexit “One of Britain’s great strengths is the ability to offer and aggregate all of the services the global financial services industry needs” “This has not changed as a result of the EU referendum and I will do everything I can to ensure the City of London retains its position as the world’s leading international financial centre.”
2/11 Yanis Varoufakis
Greece's former finance minister compared the UK relations with the EU bloc with a well-known song by the Eagles: “You can check out any time you like, as the Hotel California song says, but you can't really leave. The proof is Theresa May has not even dared to trigger Article 50. It's like Harrison Ford going into Indiana Jones' castle and the path behind him fragmenting. You can get in, but getting out is not at all clear”
3/11 Michael O’Leary
Ryanair boss says UK will be ‘screwed’ by EU in Brexit trade deals: “I have no faith in the politicians in London going on about how ‘the world will want to trade with us’. The world will want to screw you – that's what happens in trade talks,” he said. “They have no interest in giving the UK a deal on trade”
4/11 Tim Martin
JD Wetherspoon's chairman has said claims that the UK would see serious economic consequences from a Brexit vote were "lurid" and wrong: “We were told it would be Armageddon from the OECD, from the IMF, David Cameron, the chancellor and President Obama who were predicting locusts in the fields and tidal waves in the North Sea"
5/11 Mark Carney
Governor of Bank of England is 'serene' about Bank of England's Brexit stance: “I am absolutely serene about the … judgments made both by the MPC and the FPC”
6/11 Christine Lagarde
IMF chief urges quick Brexit to reduce economic uncertainty: “We want to see clarity sooner rather than later because we think that a lack of clarity feeds uncertainty, which itself undermines investment appetites and decision making”
7/11 Inga Beale
Lloyd’s chief executive says Brexit is a major issue: "Clearly the UK's referendum on its EU membership is a major issue for us to deal with and we are now focusing our attention on having in place the plans that will ensure Lloyd's continues trading across Europe”
8/11 Colm Kelleher
President of US bank Morgan Stanley says City of London ‘will suffer’ as result of the EU referendum: “I do believe, and I said prior to the referendum, that the City of London will suffer as result of Brexit. The issue is how much”
9/11 Richard Branson
Virgin founder believes we've lost a THIRD of our value because of Brexit and cancelled a deal worth 3,000 jobs: We're not any worse than anybody else, but I suspect we've lost a third of our value which is dreadful for people in the workplace.' He continued: "We were about to do a very big deal, we cancelled that deal, that would have involved 3,000 jobs, and that’s happening all over the country"
10/11 Barack Obama
US President believes Britain was wrong to vote to leave the EU: "It is absolutely true that I believed pre-Brexit vote and continue to believe post-Brexit vote that the world benefited enormously from the United Kingdom's participation in the EU. We are fully supportive of a process that is as little disruptive as possible so that people around the world can continue to benefit from economic growth"
11/11 Kristin Forbes
American economist and an external member of the Monetary Policy Committee of the Bank of England argues that the economy had been “less stormy than many expected” following the shock referendum result: “For now…the economy is experiencing some chop, but no tsunami. The adverse winds could quickly pick up – and merit a stronger policy response. But recently they have shifted to a more favourable direction”
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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