British Foreign Secretary Boris Johnson wrote an article urging that Britain stay in the EU just days before becoming the chief campaigner for Brexit, the Sunday Times newspaper reported.

In the previously secret unseen column, Johnson asserted that remaining in the European Union would be “a boon for the world and for Europe”.

He warned that Brexit could lead to “economic shock” and the breakup of the union between Scotland and the rest of the United Kingdom, in the text revealed in a new book by the newspaper’s political editor Tim Shipman, entitled “All Out War”.

Then there is “the Putin factor”, Johnson wrote, adding: “We don’t want to do anything to encourage more shirtless swaggering from the Russian leader, not in the Middle East, not anywhere.”

“There are some big questions that the ‘out’ side need to answer,” he wrote.

The column was penned just two days before the shock announcement in February that Johnson would head the “Leave” campaign, according to the Sunday Times.

It said he wrote the piece “as a way of clarifying his thoughts”, before composing a final article arguing the case for Brexit, published in the Daily Telegraph newspaper in March.

In the pro-EU column, Johnson supports membership of the European single market, which this week he described as “increasingly useless”.

“This is a market on our doorstep, ready for further exploitation by British firms: the membership fee seems rather small for all that access,” the former London mayor wrote.

“Why are we so determined to turn our back on it?”