Fifty Republican national security experts in an open letter on Monday warned that party nominee Donald Trump “would be the most reckless president” in US history.
The group, which includes the former CIA director Michael Hayden, said Mr Trump “lacks the character, values and experience” to be president.
Many of the signatories had declined to sign a similar note in March, the BBC says.
“None of us will vote for Donald Trump,” the letter states.
The letter comes after a number of high-profile Republicans stepped forward to disown Mr Trump
Mr Trump has broken with years of Republican foreign policy on a number of occasions.
The candidate has questioned whether the US should honour its commitments to Nato, endorsed the use of torture and suggested that South Korea and Japan should arm themselves with nuclear weapons.
“He weakens US moral authority as the leader of the free world,” the letter read.
“He appears to lack basic knowledge about and belief in the US Constitution, US laws, and US institutions, including religious tolerance, freedom of the press, and an independent judiciary.”
Also among those who signed the letter were John Negroponte, the first director of national intelligence and later deputy secretary of state; Robert Zoellick, who was also a former deputy secretary of state and former president of the World Bank; and two former secretaries of homeland security, Tom Ridge and Michael Chertoff.
