NEW YORK - A group of 50 senior Republican national security officials signed a letter Monday warning that presidential candidate Donald Trump “lacks the character, values and experience to be president” of the United States.

The letter, first reported by The New York Times, warns: "We are convinced that in the Oval Office, he would be the most reckless president in American history."

“The undersigned individuals have all served in senior national security and/or foreign policy positions in Republican administrations, from Richard Nixon to George W Bush…” the letter said. “We know the personal qualities required of a president of the United States. None of us will vote for Donald Trump,” though it notes later that many Americans “have doubts about Hillary Clinton, the Democratic presidential nominee, as do many of us.”

The letter says Trump would weaken the United States’ moral authority and questions his knowledge of and belief in the Constitution. It says 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. And it laments that “Mr Trump has shown no interest in educating himself.”

“From a foreign policy perspective, Donald Trump is not qualified to be president and commander-in-chief,” the letter reads. “Indeed, we are convinced that he would be a dangerous president and would put at risk our country’s national security and well-being.”

“He weakens US moral authority as the leader of the free world,” the letter continues. The group said that they would not vote for Trump, but did not expressly say they would vote for Clinton.

"In addition, Mr Trump has demonstrated repeatedly that he has little understanding of America’s vital national interests, its complex diplomatic challenges, its indispensable alliances, and the democratic values on which US foreign policy must be based.

At the same time, he persistently compliments our adversaries and threatens our allies and friends. 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’.

“We understand that many Americans are profoundly frustrated with the federal government and its inability to solve pressing domestic and international problems. We also know that many have doubts about Hillary Clinton, as do many of us. But Donald Trump is not the answer to America’s daunting challenges and to this crucial election.

"We are convinced that in the Oval Office, he would be the most reckless president in American history,” the letter added.

The letter echoed similar sentiment shared by some Republican national security officials in March, but the new additions came after Mr Trump encouraged Russia to hack Mrs Clinton's email server, according to the New York Times.

Trump later said he was "being sarcastic" when he made the remarks about hacking his rival's emails.

Missing from the letter were former secretaries of state Henry Kissinger, George Shultz, James Baker, Colin Powell and Condoleezza Rice.

In a statement, Trump called the letter "politically motivated", saying the names on the letter were "the ones the American people should look to for answers on why the world is a mess".

"We thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place," he continued.

"They are nothing more than the failed Washington elite looking to hold on to their power and it's time they are held accountable for their actions."