Donald Trump
GOP presidential candidate Donald Trump speaks at a campaign rally. He's too risky to vote for, say US senior security officials Reuters/Carlo Allegri

In a fresh blow to Donald Trump's presidential campaign, 50 former top national security officials have signed a letter stating that the Republican presidential candidate lacks the "character" and "values" to step into the White House, and would present a profound risk to the nation as commander of its nuclear arsenal.

Trump would be the "most reckless president in American history," states the unprecedented letter, signed by the former high-ranking officials, every one of them a Republican.

"He is unable or unwilling to separate truth from falsehood," the letter states. "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.

"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."

Trump is "not qualified to be president and commander-in-chief," the letter goes on. "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 warning cites the candidate's "lack of knowledge" about the "US Constitution, US laws and US institutions, including religious tolerance, freedom of the press and an independent judiciary."

He has "demonstrated repeatedly that he has little understanding" of the nation's "vital national interests, its complex diplomatic challenges, its indispensable alliances and the democratic values" on which American policy should be based, the missive adds. Yet, "Mr Trump has shown no interest in educating himself."

Most "fundamentally, Mr Trump lacks the character, values and experience to be president. He [would] weaken US moral authority as the leader of the free world."

All of the signatories have served under Republican presidents, from Richard Nixon to George W Bush, with high-profile positions in intelligence, the Department of Justice (DoJ), the State Department and as advisers to the White House.

Some of the top names include former CIA and National Security Agency director Michael Hayden, former secretaries of homeland security Tom Ridge and Michael Chertoff, and one-time deputy secretary of state Robert Zoellick, who also served as president of the World Bank. The letter states that every one of the officials who signed the missive "know the personal qualities required of a president."

The letter concludes: "None of us will vote for Donald Trump." The letter was not, however, an endorsement for Democratic presidential candidate Hillary Clinton.

"We also know that many have doubts about Hillary Clinton, as do many of us," the letter stated. "But Donald Trump is not the answer."

A similar unsigned open letter was written and circulated in March. But this time around, officials put their names to the warning because of serious new concerns about Trump's recent invitation to Russia to hack into Clinton's emails, and his stance that Nato allies have to pay up before the US will support them in the event of a military attack, sources told the New York Times.

The current letter focused on Trump's lack of qualifications to be president and his character, rather than specifics of his policies, John Bellinger III, who drafted the letter, told the NY Times. Bellinger served as legal adviser to former secretary of state Condoleezza Rice at the National Security Council and the State Department.