Washington, Aug 9 (IANS): Fifty prominent Republican national security experts have signed a letter denouncing Donald Trump's presidential candidacy and pledging not to vote for him.

"We are convinced that in the Oval Office, he would be the most reckless President in American history," CNN reported citing the letter that was issued on Monday.

Its signatories include former Central Intelligence Agency (CIA) and National Security Agency Director Michael Hayden, former Director of National Intelligence and Deputy Secretary of State John Negroponte and Eric Edelman, who was former Vice President Dick Cheney's national security adviser and has worked closely with Michele Flournoy -- a candidate for secretary of defecse in a prospective Clinton administration -- to forge a centrist group of defence experts on key military issues.

The letter also includes two Homeland Security secretaries under former President George W. Bush, Tom Ridge and Michael Chertoff, and Robert Zoellick, a former World Bank president, US trade representative and deputy secretary of state.

Trump's campaign responded with a statement from Trump denouncing the signatories as people who deserve the "blame for making the world such a dangerous place", CNN reported.

Many of the same leaders wrote an open letter in March during the Republican primaries condemning Trump and pledging to oppose his candidacy, at a time when other party candidates remained in the race.

The new letter acknowledged that many Americans "have doubts about (Democratic nominee) Hillary Clinton, as do many of us."

"But Donald Trump is not the answer to America's daunting challenges and to this crucial election," CNN quoted the letter as saying.

In the letter, the group warned Trump "lacks the temperament to be president".

"He is unable or unwilling to separate truth from falsehood. He does not encourage conflicting views. He lacks self-control and acts impetuously. He cannot tolerate personal criticism. He has alarmed our closest allies with his erratic behaviour," the letter claimed.

"All of these are dangerous qualities in an individual who aspires to be president and commander-in-chief, with command of the US nuclear arsenal."

The letter was drafted by John Bellinger, a former State Department legal advisor to then-Secretary of State Condoleezza Rice, with edits from Bob Blackwill, a former George H.W. Bush White House adviser, and Eliot Cohen, also a former advisor to Rice, CNN added.