Three leading professors of psychiatry have written to Barack Obama to express their “grave concern” over Donald Trump’s mental stability.

In the letter addressed to the US president, doctors from Harvard Medical School and the University of California have urged him to order a “full medical and neuropsychiatric evaluation” before the President-elect takes office in January.

The group said it could not speculate on a diagnosis, but Mr Trump’s “grandiosity, impulsivity, hypersensitivity to criticism” led them to believe he was unfit for office.

“Professional standards do not permit us to venture a diagnosis for a public figure whom we have not evaluated personally,” the letter signed by Judith Herman, Nanette Gartrell and Dee Mosbacher, and published by the Huffington Post, reads.

“Nevertheless, his widely reported symptoms of mental instability — including grandiosity, impulsivity, hypersensitivity to slights or criticism, and an apparent inability to distinguish between fantasy and reality — lead us to question his fitness for the immense responsibilities of the office.”

In August, President Obama questioned Mr Trump's judgement and temperament and called him “unfit to serve as president.”

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“I said so last week and he keeps on proving it. The notion that he would attack a Gold Star family that had made such extraordinary sacrifices on behalf of our country, the fact that he doesn’t appear to have basic knowledge around critical issues in Europe, in the Middle East, in Asia means that he’s woefully unprepared to do this job,” he said.

“There has to come a point at which you say somebody who makes those kinds of statements doesn’t have the judgment, the temperament, the understanding to occupy the most powerful position in the world because a lot of people depend on the White House getting stuff right.”

According to the American Psychiatric Association (APA), there are nine criteria for “Narcissistic Personality Disorder” - with a diagnosis being offered if an individual has five or more of the listed qualities.

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The APA summarise the condition as: “A pervasive pattern of grandiosity (in fantasy or behavior), need for admiration, and lack of empathy, beginning by early adulthood and present in a variety of contexts.”

This is not the first time mental health professionals have weighed in on Mr Trump’s suitability for office.

In June, Atlantic published a psychologist’s findings that Mr Trump suffered from "narcissism, disagreeableness, grandiosity".

Mr Trump's well-documented "no-holds-barred" approach to the US presidency has led many to question his diplomatic suitability.

The real-estate mogul regularly takes to Twitter to post provocative statements aimed at his rivals, often breaking with long-established foreign policy protocol.

On Saturday, he accused China of “stealing” in a tweet that caused an immediate backlash from Beijing.

Initially misspelling "unprecedented," Mr Trump tweeted: "China steals United States Navy research drone in international waters - rips it out of water and takes it to China in unpresidented act."

He later deleted and then reposted the tweet to correct the spelling.

In response, China accused Washington of "hyping up" the issue.

APA protocol means medical professionals should not diagnose individuals they have not personally treated.