Former US president George H W Bush voted for Hillary Clinton in the 2016 election and called Donald Trump a "blowhard" who was driven by "a certain ego."

His son George W. Bush left his ballot blank and said about Trump: "This guy doesn't know what it means to be president."

The revelations, contained in historian Mark Updegrove's "The Last Republicans," available in bookstores later this month, represent the strongest criticism to date from the Bush clan on why their Republican successor is uniquely unfit to hold office.

"I don't like him," the elder Bush told Updegrove in May 2016 before the elections in November of that year, according to excerpts of the book obtained by AFP.

"I don't know much about him, but I know he's a blowhard. And I'm not too excited about him being (our) leader."