Nick MacKinnon, who taught the PM at posh private £49,000-a-year Winchester College, slammed his "selfserving, opportunistic" former pupil for "the way he turned out".
And he vowed to vote for Keir Starmer's party to punish Mr Sunak for his "completely unforgivable" backing of Brexit, which he says has put the country back 20 years.
The maths tutor and ex-housemaster told this newspaper: "I don't know how he sleeps at night." The criticism comes despite the Prime Minister, 44, and his wife Akshata Murty donating more than £100,000 to the school he attended from the age of 13 to 18 and left in 1998.
Mr MacKinnon said: "I remember him only as a boy at the back of class.
"I recall little about him as a pupil but his achievements at Winchester are not in dispute. He was a very good boy and I am sorry for the way he has turned out, which was to become wildly opportunistic." He added: "I will be voting for Labour against Sunak. In my mind he has to be punished by my one tiny little vote, but that's what I will do.
"It is a punishment vote for Brexit. If I'd done what he did over Brexit, I'd be struggling to get to sleep every night.
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