Leading professionals have been told they may discuss Donald Trump's mental health, because of his unprecedented behaviour.

Psychiatrists are traditionally bound by a rule that prevents them from talking about the mental wellbeing of public figures, or diagnosing them from afar. But the new ruling from the American Psychoanalytic Association suggests that rule is breaking down because of the President's behaviour.

A statement emailed to the group's 3,500 members said the leader's actions had led to a re-consideration of what is referred to as the Goldwater Rule.

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The letter was written because of a "belief in the value of psychoanalytic knowledge in explaining human behaviour", ex-president Prudence Gourguechon told Stat News.

"We don’t want to prohibit our members from using their knowledge responsibly," she said.

The decision had been made amid growing concern about Donald Trump's mental health, Dr Gourguechon said. The organisation chose to lift the rule because "Trump’s behaviour is so different from anything we’ve seen before", Stat News reported.

The American Psychiatric Association, a much larger professional group that was responsible for formulating the Goldwater Rule, still expects its members to comply with it. It reiterated that in March, amid growing questions about whether its members would be allowed to discuss the president's mental state.

The rule is named after Barry Goldwater, and came amid speculation about whether he was mentally healthy enough to run for president. Since that time, psychiatrists are told not to speculate about any particular person to avoid a repeat of such speculation, which was deemed by professionals to be unethical.

But in truth the Goldwater Rule is little enforced anyway. Theoretically, professional groups can complain to member's medical boards, but that is thought never to have actually happened.