The Vice-president of Republicans Overseas Dr Jan Halper-Hayes has lashed out the GOP presidential nominee Donald Trump, calling the billionaire "truly psychologically unbalanced".

Speaking on BBC Radio 4's Today programme, Hayes said that while she felt guilty for criticising the head of her party, Trump had "gone over the top".

Hayes explained she had the property mogul and former reality TV star fill out a temperament assessment for her book Quiet Desperation: The Truth About Successful Men.

She said his temperament, the personality traits he was born with, were "really concerning" and added "Donald is out of control right now".

Trump, who has courted unprecedented controversy since he began his bid for the US presidency, has had to defend comments in which he attacked the parents of a Muslim soldier US Army Capt Humayun Khan, killed in action in Iraq in 2004.

At the Democratic National Convention the soldier's father, Khizr Khan, had denounced Trump for his pledge to ban Muslims from entering the United States if elected president. Republican figures including Senator John McCain, the 2008 Republican presidential nominee have criticised Trump for his attacks on the family.

The son-in-law of Republican firebrand Sarah Palin, Dakota Meyer, who received the Medal of Honor in 2011, has also lashed out at Trump on Twitter.

"If @realDonaldTrump wants to be the Commander in Chief, he needs to act like one. And that cant start until he apologizes to the Khans," Meyer said on the social media platform.

Donald Trump
Republican presidential nominee Donald Trump speaks to voters during a campaign event in Ashburn, Virginia Getty

Note (25 August 2016): This article has been amended to remove reference to the organisation's constituency.