The contents of a previously unpublished pro-EU article written by Boris Johnson two days before he publicly announced his support for the Leave camp, have been made public.

The existence of the article, which Johnson wrote for The Daily Telegraph, was well known, but the contents were not - until they were shared in the Brexit book 'All Out War', by Tim Shipman, the political editor of The Sunday Times.

In his pro-EU article, the book alleges Johnson argued Brexit could cause the breakup of Britain and warned that leaving could lead to "economic shock", Sky News reported, suggesting some of the contents of the article contradict things Johnson has said in his role as foreign secretary.

"This is a market on our doorstep, ready for further exploitation by British firms," Johnson allegedly wrote on 19 February.

"The membership fee seems rather small for all that access. Why are we so determined to turn our back on it?"

The book also states Johnson wrote: "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."

Just two days later, Johnson declared his support for the Leave campaign, but it is reported he penned the pro-EU copy to get his thoughts on the referendum in order, prior to writing the second of two published columns for The Daily Telegraph supporting Brexit.

All Out War also alleges Johnson "wanted to punch" Michael Gove for his surprise bid to run for leadership of the Conservative Party, and suggests Johnson would have become defence secretary if David Cameron had remained prime minister.

The release of the column's contents comes as UK prime minister Theresa May is preparing to meet with European Commission president Jean Claude Junker next week to discuss immigration control and post-Brexit trade.