Michael Wolff's controversial book on sale in California | Justin Sullivan/Getty Images Special relationship in doubt if Trump doesn’t get royal wedding invite, says Wolff ‘Fire and fury’ author says US president wants to outshine the Queen — and didn’t know what Brexit was.

A post-Brexit trade deal between the U.K. and U.S. could be in danger if Donald Trump doesn't get invited to the wedding of Prince Harry and Meghan Markle, according to the author of a controversial new book about the U.S. president.

Michael Wolff told the Mail on Sunday that Trump would resent any snub and the so-called special relationship could be damaged unless he "gets what he wants."

Trump "doesn't like being snubbed and wants to be the center of attention all the time," Wolff said. "Trump's foreign policy doctrine is simple: You Brits suck up to him and enlist in whatever geopolitical fantasy he has going, he'll give you what you want — though only if it doesn't hurt him. It is not so much vengeance, rather 'you flatter me and I'll flatter you.'"

"If the Brits give him what he wants he will value the Brits," Wolff said.

Trump is thought unlikely to be invited to the royal wedding because Prince Harry is friends with Barack Obama.

However, Trump is due to visit the U.K. in 2018 for a state visit. According to Wolff, the president's main aim is to meet — and outshine — the Queen.

"He sees the Queen in reality TV show terms. That's the Trump modus operandi. He will try to Trumpalize the Queen and Buckingham Palace."

Wolff, whose book "Fire and Fury: Inside the Trump White House" has been dismissed by Trump as "boring and untruthful," also claimed that Trump did not know what Brexit was at the time of summer 2016 referendum.

"I interviewed Trump two weeks before your 2016 referendum and he had no idea what Brexit was," Wolff told the Mail.

Suggestions in the book that Trump is mentally unfit to be president were out of “the old Ronald Reagan playbook,” Trump tweeted on Saturday.

“Actually,” the president said, “throughout my life, my two greatest assets have been mental stability and being, like, really smart.”

He also said he “would qualify as not smart, but genius ... and a very stable genius at that!”