Several prominent professors of psychiatry affiliated with prestigious American universities have written an open letter to the US Congress asking for the “lawful” removal of US President Donald Trump.

In an open letter to Congress published by the Huffington Post, the five eminent professors observe that the current US president is unfit to serve, based on what they state is Trump's mental "unraveling," the Washington Times reported.

The letter from the psychiatry notables is uncharacteristically blunt.

"The power of the presidency has accentuated Mr. Trump's failings with devastating effects," the letter states, adding that the president's "psychological isolation — including his tenuous relationship to reality — has led to the dangerous isolation of the United States from the rest of the world."

"Our Constitution has provisions for removing any president who is unable to discharge the duties of office," the professors observe.

"For the sake of our country and the world, we urge our elected representatives to summon their courage so that lawful steps can be taken to end the Trump presidency."

The letter is signed by Judith L. Herman of Harvard, Robert Jay Lifton of Columbia University, Bandy X. Lee of Yale, and Dee Mosbacher and Nanette Gartrell of the University of California, according to the the Washington Times.

The letter follows a November 2016 warning from Herman, Mosbacher and Gartrell to former President Barack Obama, in which they expressed a "grave concern" over Trump's mental acuity, pointing to what they diagnosed as a severe case of Narcissistic Personality Disorder.