Mr Hastings got know Mr Johnson – who is running against Jeremy Hunt for the Tory leadership – in the 1980s, when he was editor of the Daily Telegraph. Mr Hastings wrote in the Guardian today: “I have argued for a decade that, while he is a brilliant entertainer who made a popular maître d’ for London as its mayor, he is unfit for national office, because it seems he cares for no interest save his own fame and gratification. “We can’t predict what a Johnson government will do, because its prospective leader has not got around to thinking about this.
“But his premiership will almost certainly reveal a contempt for rules, precedent, order and stability.”
Mr Hastings, who described his former employee as “morally bankrupt”, said Mr Johnson would never have the dignity required for public office.
He added: “Yet his graver vice is cowardice, reflected in a willingness to tell any audience, whatever he thinks most likely to please, heedless of the inevitability of its contradiction an hour later.”
Mr Hastings also castigated Mr Johnson for his “weak character”.
He said: “I recently suggested to a radio audience that he supposes himself to be Winston Churchill, while in reality being closer to Alan Partridge.
“Churchill, for all his wit, was a profoundly serious human being.
“Far from perceiving anything glorious about standing alone in 1940, he knew that all difficult issues must be addressed with allies and partners.”
Mr Churchill was a compassionate man, whereas Mr Johnson “confined compassion to himself”, Mr Hastings said.
He added: “He has long been considered a bully, prone to making cheap threats.
“I have handwritten notes from our possible next prime minister, threatening dire consequences in print if I continued to criticise him.
“Johnson would not recognise truth, whether about his private or political life, if confronted by it in an identity parade.
“Almost the only people who think Johnson a nice guy are those who do not know him.”
Nevertheless, the incompetence of Labour leader Jeremy Corbyn boosted Mr Johnson’s hopes of staying in Number 10, Mr Hastings said.
He explained: “He may yet prove to be the only possible Labour leader whom Johnson can defeat in a general election.
“If the opposition was led by anybody else, the Tories would be deservedly doomed, because we would all vote for it.
“As it is, the Johnson premiership could survive for three or four years, shambling from one embarrassment and debacle to another, of which Brexit may prove the least.
“I have a hunch that Johnson will come to regret securing the prize for which he has struggled so long, because the experience of the premiership will lay bare his absolute unfitness for it.
“If the Johnson family had stuck to show business like the Osmonds, Marx Brothers or von Trapp family, the world would be a better place.
“Yet the Tories, in their terror, have elevated a cavorting charlatan to the steps of Downing Street, and they should expect to pay a full forfeit when voters get the message.”