German MEP Manfred Weber made the astonishing comments on Twitter after a copy of an unpublished column written by Mr Johnson arguing in favour of Remain emerged during the weekend.
The former London Mayor and Vote Leave figurehead appeared to contradict himself by writing in the column that Britain’s EU exit would be “a boon for the world and for Europe”, even claiming the cost of EU membership was “rather small”.
The exposure of Mr Johnson’s column, written just days before he decided to defy his Prime Minister’s stance on Brexit, enraged the German politician and head of the EPP group, who tweeted: “Boris Johnson is an unscrupulous careerist and opportunist without conviction. If he has any decency, he will step down.”
Express.co.uk contacted the Foreign Office for a response to Mr Weber’s strongly worded outburst, but have yet to receive a reply.
Mr Johnson has sprung to his own defence regarding the pro-Remain column, written in February, claiming he was still “wrestling” with the decision and that the article was “semi-parody”.
The article for the Daily Telegraph, concluded: “Yes, folks, the deal”s a bit of a dud, but it contains the germ of something really good. I am going to muffle my disappointment and back the Prime Minister.”
The 52-year-old Tory said the column was part of the process he went through to decide which side he would commit to regarding Brexit.
He said: “Everybody was trying to make up their minds about whether or not to leave the European Union and 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 so I then wrote a sort of semi-parodic article in the opposite sense. I set them side by side and it was blindingly obvious what the right thing to do was.”
Mr Johnson has received support online from his Vote Leave compatriot Michael Gove, who tweeted: “I am in no doubt @BorisJohnson is and was 100 per cent sincere and passionate about Britain leaving the EU – his conviction was clear.
“It is never easy to take a different line from your party leader – especially on such a big issue – but @BorisJohnson followed his heart.
“(He) acted in accord with his deep beliefs – his motivation was to do what he thought was right for the UK long-term.”
But Mr Johnson is reported to have rejected an offer of renewed friendship from Mr Gove following the Scot’s backstabbing antics during the Tory leadership contest.
Journalist Tim Shipman, whose new book on Brexit reveals the unpublished column, claims the Foreign Secretary “wanted to punch” Mr Gove for betraying him.