Donald Trump is “dangerous” and drastic steps must be taken to protect the public from him, two leading psychiatrists have warned.

The President’s erratic behaviour, including “repeated failure to distinguish between reality and fantasy” and “paranoid claims of conspiracy”, cast doubt over his ability to react rationally in a crisis, they said.

In a letter to the New York Times, Judith Herman, a professor of psychiatry at Harvard Medical School, and Robert Lifton, a lecturer in psychiatry at Columbia University, said they were not attempting to diagnose Mr Trump.

“We are in no way offering a psychiatric diagnosis, which would be unwise to attempt from a distance," they wrote.

"Nevertheless, as psychiatrists we feel obliged to express our alarm. We fear that when faced with a crisis, President Trump will lack the judgment to respond rationally.

“The military powers entrusted to him endanger us all. We urge our elected representatives to take the necessary steps to protect us from this dangerous president.”

Last month, 35 mental health professionals wrote to the newspaper warning the “grave emotional instability” indicated by Mr Trump’s behaviour made him incapable of serving safely as President.

This drew some criticism as it is usually frowned upon among psychiatrists to give a professional opinion of the mental state of someone they have not examined in person, as dictated by a passage in the American Psychiatric Association’s code of ethics known as the Goldwater rule.

Stephen Colbert on Trump's wiretapping claims: Someone get this guy a Xanax

Professor Herman and Dr Lifton gave the President’s unsubstantiated claim that Barack Obama ordered a wiretap of his phones during the presidential campaign as an example of his irrational behaviour.

“Even within the space of a few weeks, the demands of the presidency have magnified his erratic patterns of behaviour,” they wrote.

“We are struck by his repeated failure to distinguish between reality and fantasy, and his outbursts of rage when his fantasies are contradicted. Without any demonstrable evidence, he repeatedly resorts to paranoid claims of conspiracy.

“Most recently, in response to suggestions of contact between his campaign and agents of the Russian government, he has issued tirades against the press as an “enemy of the people” and accusations without proof that his predecessor, former President Barack Obama, engaged in partisan surveillance against him.”

Harvard Law School professor Noah Feldman said if Mr Trump’s allegations regarding Mr Obama were proved false, a major scandal would arise that could lead to his impeachment.

The President recently posted a series of early-morning tweets in which he accused his predecessor of ordered the wiretap.

“Terrible! Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!” he wrote.

“Is it legal for a sitting President to be “wire tapping” a race for president prior to an election? Turned down by court earlier. A NEW LOW!”

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“How low has President Obama gone to tapp [sic] my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!”

Mr Trump did not provide any evidence for his claims and his spokespeople have consistently refused to do so.