BORIS Johnson warned an English-only vote for Brexit could lead to Scottish independence and the break-up of the Union, a previously secret newspaper column has revealed.

Written in February, two days before he backed the Leave side, the unpublished article also said remaining in the EU would be a “boon for the world and for Europe”.

Brexit could also lead to an “economic shock” and Russian aggression, Mr Johnson said.

Read more: Nicola Sturgeon to publish plan for Scotland to stay in single market after Brexit

The existence of the pro-Remain column had been widely known, but its content was revealed by the Sunday Times in an extract from a new book on Brexit, All Out War.

The Foreign Secretary yesterday defended writing the “semi-parodic” piece - which he ultimately rejected in favour of a pro-Leave column published in the Telegraph - explaining he had been “wrestling” with his position .

Setting the two columns side by side, he said it had been “blindingly obvious what the right thing to do was”.

He said: “It is perfectly true that back in February 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 favour of leaving. I then thought I better see if I can make the alternative case for myself.

“I think the people made the right decision, they voted very substantially to leave the European Union, that is what we're going to do and we're going to make a great success of it."

Read more: Nicola Sturgeon to publish plan for Scotland to stay in single market after Brexit

Although nominally pro-Remain, the column is filled with criticism of the deal David Cameron struck with EU leaders earlier this year, and other complaints about Brussels.

It also covered the ramifications of Brexit at home and abroad.

Mr Johnson wrote: “There is the worry about Scotland, and the possibility that an English-only ‘leave’ vote could lead to the break-up on the Union.

“There is the Putin factor: we don’t want to do anything to encourage more shirtless swaggering from the Russian leader, not in the Middle East, not anywhere.”

Despite Mr Johnson last week saying the UK could swap the “increasingly useless” EU single market for a trade deal “of greater value” with Brexit, he praised the former in February.

“This 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?”

Former LibDem Deputy Prime Minister Nick Clegg said Mr Johnson and other “opportunists and chancers” in the Leave camp had lied to the country about the impact of Brexit.

He told the BBC’s Andrew Marr show: “If I was a Brexit voter, I would feel increasingly betrayed that I voted in the belief that all these Brexiteers knew what they were doing.

Read more: Nicola Sturgeon to publish plan for Scotland to stay in single market after Brexit

“I would be increasingly angry - particularly as my electricity and gas prices go up, as I realise I can’t take my kids on that holiday to Spain because it is now 20 per cent more expensive.”

Mr Clegg, a member of a cross-party group of MPs trying to force a Commons vote on what kind of Brexit the UK should advocate, said it would be a “good thing” if Theresa May was defeated and had to delay withdrawal talks with the EU beyond her March 2017 deadline.

She would have to “go back and improve her negotiating stance” if that happened, he said.

Pro-Brexit International development Secretary Prit Patel warned MPs not to use Parliament “as a vehicle to subvert the democratic will of the British people".

She said the Commons was already discussing the government's strategy on a daily basis.

Lucy Thomas, former director of the Stronger In campaign, said the column showed Mr Johnson’s decision to back Leave had been based on political calculation, not belief.

“None of it is in the detail, none of it is about what life outside the EU look like,” she said.