Top Tory Brandon Lewis retweeted a meme branding Boris Johnson a "cold, calculating narcissist" and a "sociopath" who will "screw the UK over."
It comes as Conservative Party faithful descended on Manchester for the first full day of the party's annual conference.
But the bash got off to a rocky start, with Mr Johnson hit by fresh cash for access claims after donors were offered a cosy dinner with a senior minister for £4,000.
And the Sunday Mirror revealed a quarter of ministers attending Mr Johnson new cabinet had accepted donations from three wealthy and well-connected donors with links to Russia.
This morning, the Northern Ireland Secretary, apparently accidentally, retweeted the image, which featured a picture of the Prime Minister staring into the camera.
The text reads: "Don't be fooled. Boris is a cold, calculating narcissist, a sociopath with the moral empathy of bacteria.
"His bumbling awkwardness is practiced, like a Music Hall drunk. Don't ever trust him. He will screw the UK over without breaking into a sweat."
The original tweet was sent last night by an account under the name "Narcopath Info", and retweeted this morning by Mr Lewis.
Shortly before retweeting the unfortunate meme, Mr Lewis had wished people running the London Marathon today good luck.
The Conservative Party declined to comment.
The embarrassing foul up took place just as the Prime Minister was starting his headline interview on the BBC's Andrew Marr programme.
Mr Johnson was seen earlier this morning going for a run in a dress shirt and black shoes.
In his interview with Andrew Marr, the PM failed to rule out hiking taxes in future, saying he would oppose "unnecessary" tax rises, and that he would not agree to them if he "can possibly avoid it"
