In a newspaper column drafted in February, and published by the Sunday Times, he suggested staying in the EU would be a "boon for the world".
UK Socialist MEP Richard Corbett said, "This confirms suspicions that Boris Johnson swung behind the Leave campaign, not because he believed in it, nor even expecting it to win, but simply to boost his appeal to the Eurosceptic wing of the Conservative party, miscalculating that this would boost his prospects to lead the party."
His colleague David Martin told this website, "We now have confirmation that Boris means Boris, and that he had no real views on the UK's membership of the EU, just a strong sense of what was good for Boris politically.
"As foreign secretary he is behaving in the same manner. He knows hard Brexit is a disaster for the UK economy but wants to appear like a resolute politician rather than the political jelly he really is."
Ukip MEP Julia Reid said, "Reading Boris's article, it was clear that his support for Brexit was authentic and well-thought out, quite to the contrary of what many people have suggested."
"When will the remoaners stop telling the electorate why they voted Leave and help us get on making Brexit a success for Britain? Ukip will work for a democratic Brexit, the real deal, the full lunch that the British people asked for."
Further criticism came from Andrew Duff, a former Liberal Democrat MEP and now at the European Policy Centre, a Brussels think tank.
Duff said that, in writing the article and then supporting the Leave camp, Johnson had shown himself to be "more a complete parody of himself than 'semi-parodic'."
Former Liberal MEP and ex British deputy Prime Minister Nick Clegg, who supported a Remain vote, suggested Johnson and other "opportunists and chancers" backing Brexit had lied to the British people during the referendum campaign.
Elsewhere, SNP leader Nicola Sturgeon said, "Let me be crystal clear about this - Scotland cannot trust the likes of Boris Johnson to represent us."
Denis Macshane, the UK's former Europe minister under Tony Blair, said, "In the world of politics words mean something. In the world of Boris words mean nothing. He sees himself as a Harry Houdini of his own mouth and pen ever able to escape responsibility for his weather vane political utterances.
"It scarcely matters save that Johnson is Her Majesty's principal secretary of state for foreign affairs and that used to be a job in which words were weighed carefully as an utterance by the British foreign secretary once mattered in the world.
"Now everyone knows that Boris has his fingers crossed behind his back whenever he speaks or writes and that was he says today will be inoperative tomorrow.
"It may amuse some but the end result is that the most cynical of all the hardline Brexiters is doing serious damage to his country as every time Boris opens his mouth, someone somewhere sells sterling or decides that Britain does not have a serious government whose word can be trusted."
Boris Johnson defended his writing of a pro-EU article days before he publicly backed Brexit, saying the article was "semi-parodic" and the UK's decision to leave was right.
He said he was "wrestling with the issue" at the time and was merely trying to make the "alternative case".
Johnson was a leading figure in the campaign to exit the EU and became foreign secretary after the Leave vote in the June referendum.
Amid growing pressure on the UK government to spell out its negotiating objectives for Brexit, Johnson insisted last week that the UK could get a trade deal that was "of greater value" to the economy than access to the EU single market, which he described as an "increasingly useless" concept.
But in February's pro-Remain column, Johnson backed membership of the free trade zone, describing it as "a market on our doorstep, ready for further exploitation by British firms."
In the article, which was produced at the same time as a pro-Brexit article published in the Telegraph, Johnson also warned Brexit could lead to an economic shock, Scottish independence and Russian aggression.
He wrote, "There are some big questions that the 'out' side need to answer. Almost everyone expects there to be some sort of economic shock as a result of a Brexit."