Boris Johnson's anti-Brexit article published for first time

In an embarrassment to Boris Johnson, a leading daily in the UK on Sunday made public the British Foreign Secretary's previously unpublished column in which he had appealed to Britain to stay in the EU, days before becoming the star campaigner for Brexit.

In the pro-remain column for The Sunday Times, published for the first time on Sunday, Johnson wrote that remaining in the European Union would be "a boon for the world and for Europe".

The Sunday Times said he wrote the piece "to clarify his thoughts", before composing a final article arguing the case for Brexit, published in the Daily Telegraph newspaper in March.

The 52-year-old former London mayor, who led the Brexit campaign before becoming foreign secretary under British Prime Minister Theresa May, declared: "Britain is a great nation, a global force for good. It is surely a boon for the world and for Europe that she should be intimately engaged in the EU."

He also warned that Brexit could lead to an economic shock, Scottish independence and Russian aggression.

"We don't want to do anything to encourage more shirtless swaggering from the Russian leader, not in the Middle East, not anywhere," he said.

"There are some big questions that the 'out' side need to answer," he wrote.

He has previously admitted to writing the piece but its contents had not been known.

The article was revealed in a new book, 'All Out War', by the newspaper's political editor Tim Shipman.

Johnson wrote the column on February 19, just two days before shocking former British Prime Minister David Cameron by opting publicly for the "leave" campaign.

He had already penned one piece arguing the case for "out", then wrote the "remain" article as a way of clarifying his thoughts, before composing a final article backing Brexit for publication.

The book dispels the myth that Johnson's case for "remain" was better than his argument to "leave".

In fact the article was dashed off quickly and seems to be an attempt by Johnson to convince himself the case for staying in was weak. But it nonetheless shows some of the concerns he had about leaving the EU.

Johnson, now a proponent of a "hard Brexit" that would take Britain out of the European single market, put the opposite argument in his "remain" column.

"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 wrote.

The book also challenges the idea that Johnson was motivated solely by his ambition to be prime minister.

Johnson's campaign to leave the EU ultimately ended in Britain voting in favour of Brexit by 52 to 48 per cent on June 23.