Donald Trump is facing another revolt from within his own party.

A group of 50 Republican national security experts have signed an open letter, saying the party's nominee "would be the most reckless president" in US history.

They say Mr Trump "lacks the character, values and experience" to be president.

The group includes two former secretaries of homeland security a former CIA director Michael Hayden.

Sky's Cordelia Lynch in Washington D-C says he's expected to formally throw his hat into the ring.