Boris Johnson has admitted he “deeply regrets” going down the Brexit route according to sources close to the former Foreign Secretary.

Matt Kelly, editor of The New European, revealed on Twitter that someone who knows Johnson very well said he now wishes he had sent the other letter.

The source said: “He knows he’s fucked up massively. Now he’s working out how to get himself out of the mess.”

Johnson penned two articles ahead of the Brexit vote before he shocked Prime Minister David Cameron by revealing he would be campaigning to leave the European Union.

In the unpublished Remain-backing article, revealed in a new book and published in The Sunday Times, he supported membership of the free trade zone.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?”

In a column wrote before the referendum was announced Johnson also advised fellow ministers to stop blaming Brussels for all our problems, saying if Britain left the EU “we would have to recognise that most of our problems are not caused by Brussels”.

He said: “Why are we still, person for person, so much less productive than the Germans? That is now a question more than a century old, and the answer is nothing to do with the EU. In or out of the EU, we must have a clear vision of how we are going to be competitive in a global economy.”