A group of 50 high-profile former officials in Republican administrations have signed a letter denouncing Donald Trump's candidacy, warning he would be "a dangerous president and would put at risk our country's national security and well-being."

The letter, signed by top foreign policy aides from the administrations of Presidents Richard Nixon through George W. Bush, is the latest in a series of warnings from prominent establishment figures against the GOP nominee.

"From a foreign policy perspective, Donald Trump is not qualified to be president and commander-in-chief," they wrote. "Indeed, we are convinced that he would be a dangerous president and would put at risk our country's national security and well-being."

The signatories include two former secretaries of the Department of Homeland Security, Michael Chertoff and Tom Ridge; former director of the CIA and the National Security Agency Michael Hayden; former Director of National Intelligence John Negroponte, two former U.S. trade representatives, Carla Hills and Robert Zoellick, and former ambassador to NATO William Taft IV.

"None of us will vote for Donald Trump," they wrote, criticizing his lack of "character, values and experience to be president."

"We also know that many have doubts about Hillary Clinton, as do many of us," they said. "But Donald Trump is not the answer to America's daunting challenges and to this crucial election."

"He weakens U.S. moral authority as the leader of the free world," the letter reads. "He appears to lack basic knowledge about and belief in the U.S. Constitution, U.S. laws and U.S. institutions, including religious tolerance, freedom of the press and an independent judiciary."

The letter is similar to one signed by more than a hundred members of the foreign policy establishment back in March who hoped to discourage Republican voters from nominating Trump. But many of the biggest names who refused to speak out against Trump during the primary told The New York Times they had changed their minds after hearing the nominee encourage Russia to find Clinton's missing emails – a comment he later said was meant as a joke.

"Unlike previous presidents who had limited experience in foreign affairs, Mr. Trump has shown no interest in educating himself. He continues to display an alarming ignorance of basic facts of contemporary international politics. Despite his lack of knowledge, Mr. Trump claims that he understands foreign affairs and 'knows more about ISIS than the generals do,'" according to the letter, which uses a nickname in referring to the Islamic State group.

None of the living former secretaries of state in Republican administrations – Henry Kissinger, George Shultz, James Baker, Colin Powell and Condoleezza Rice – joined in condemning Trump, but neither have any of them endorsed his candidacy.

Among the letter's signatories are many former top State Department aides, indicating the unease with Trump goes to some of the highest levels of the foreign policy community.