Writer Martin Amis has described Labour leader Jeremy Corbyn as "a joke" and said he would leave Labour "undeserving of a single vote".
In a highly critical article for the Sunday Times, Money author Amis said the Islington North MP was a "fluky beneficiary of a drastic elevation".
His intervention comes as Mr Corbyn faces challenges from within his party, as a Labour MP said he would try to oust him as leader if the party flopped in May's devolved and local elections.
Amis, who said he spent his 20s "close to the epicentre of the Corbyn milieu" while at the New Statesman, also called Mr Corbyn "humourless".
The 66 year old, son of late author Sir Kingsley Amis, said: "Many journalists have remarked on this, usually in a tone of wry indulgence. In fact it is an extremely grave accusation, imputing as it does a want of elementary nous. To put it crassly, the humourless man is a joke - and a joke he will never get."
"He is undereducated. Which is one way of putting it. His schooling dried up when he was 18, at which point he had two E-grade A-levels to his name; he started a course at North London Polytechnic, true, where he immersed himself in trade union studies, but dropped out after a year. And that was that.
"In general, his intellectual CV gives an impression of slow-minded rigidity; and he seems essentially incurious about anything beyond his immediate sphere."
Mr Corbyn was "without the slightest grasp of the national character", he added, saying the opposition leader's proposal to leave Nato would "paralyse" the special relationship between the UK and US.
The Booker Prize winner also said Mr Corbyn was "obviously unelectable" and predicted Labour could become a "leftist equivalent to the American GOP (Republican Party): hopelessly retrograde, self-absorbed, self-pitying and self-righteous, quite unembarrassed by its (years-long) tantrum, necessarily and increasingly hostile to democracy, and in any sane view undeserving of a single vote".
Meanwhile, Rochdale MP Simon Danczuk - one of the left-winger's most vocal internal critics - told the Mail on Sunday: "If the results for Labour in May are as dire as we all fear, then yes I would be prepared to stand as a stalking horse against Jeremy Corbyn."