Donald Trump would be a "dangerous President" and "lacks the character, values, and experience" to be America's Commander-In-Chief, according to 50 Republican security experts.

In a damning open letter signed by several prominent Republicans who have served in senior national security and/or foreign policy positions, Trump was described as "lacking self-control and acting impetuously".

The signatories, which include the former CIA director Michael Hayden, all said that they will not be voting for Trump.

The letter adds: "He weakens US moral authority as the leader of the free world.

"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."

In a statement in response to the letter, Trump said all of the people who had signed it "are the ones the American people should look to for answers on why the world is a mess".

The letter came as Republican Senator Susan Collins confirmed that she will not be voting for Trump because he is "unworthy of being President".

In a Washington Post opinion piece she wrote: "Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country."