George Snr. and George W. blast Donald Trump

The Bush presidents blast Trump in tell-all book. Photo / AP

Former President George H.W. Bush has publicly rebuked Donald Trump: "He's a blowhard."

His son, former President George W. Bush, also had some harsh words for his Republican successor: "This guy doesn't know what it means to be president."

The comments mark the first time the former presidents have spoken about Trump in such start terms. They come off the back of a new book about the father and son by historian Mark Updegrove, titled The Last Republicans.

Both men went on the record to give Mr Updegrove their candid assessment of Trump, as well as rare insight into their thoughts on the 2016 presidential race as the drama unfolded.

The most critical comments about Trump came from the elder Bush, now 93, who told Updegrove in May 2016, "I don't like him. I don't know much about him, but I know he's a blowhard. And I'm not too excited about him being a leader."

The author went on to ask George H.W. Bush what he thought Trump was seeking in running for president, and the former Navy war hero responded by saying that Trump had "a certain ego."

Neither of the two Republican former presidents voted for Trump - the father voted for Hillary Clinton and the son voted for "none of the above," as he told Updegrove.

In the book, Updegrove writes that the younger Bush did not think Trump would win when he first entered the presidential race. "Interesting, won't last" was the reaction that George W. Bush had in those days, Updegrove writes.

However, at one point during the 2016 presidential campaign, the younger Bush confided to the author, "I'm worried that I will be the last Republican president."

The White House hit back quickly.

"If one presidential candidate can disassemble a political party, it speaks volumes about how strong a legacy its past two presidents really had," a White House official told CNN. "And that begins with the Iraq war, one of the greatest foreign policy mistakes in American history."