In a previously unpublished column he penned in February, Johnson -- now the UK's foreign secretary
-- urged Britain to stay in the EU, just two days before he came out against the Remain campaign.
"Britain is a great nation, a global force for good," he wrote in the column published by The Sunday Times
. "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
would cause an "economic shock" that could lead to the "breakup" of the United Kingdom
"Shut your eyes. Hold your breath. Think of Britain. Think of the rest of the EU," he wrote. "Think of the future, 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."
Despite his doubts, Johnson went on to become a leading figure in the campaign to leave the EU, which won a tight victory in June. Prime Minister Theresa May
appointed him foreign secretary
Speaking to reporters outside his home Sunday, Johnson made light of the column, insisting he wrote it to test his resolve.
"I was wrestling with it like I think a lot of people in this country, and I wrote a long piece which came down overwhelmingly in favor of leaving," he said, according to CNN affiliate ITN.
"I then thought if I can make an alternative case to myself, wrote a kind of semi-parodic article in the opposite sense, which has mysteriously found its way into the papers this morning, because I think I might have sent it to a friend."
Johnson, talking to members of Parliament last week, insisted the UK could 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," ITN reported.
In Johnson's pro-EU column, however, he supported membership of the free trade zone.
"This is a market on our doorstep ready for further exploitation by British firms. The membership fee seems rather small for all that access," he wrote. "Why are we so determined to turn our back on it?"