The author of an explosive book on the Trump administration said the President “doesn’t like being snubbed and wants to be the centre of attention all the time”.
Michael Wolff, who has written the bestselling Fire and Fury: Inside The Trump White House, told the Mail on Sunday the 45th President’s approach to foreign policy was “simple”.
He said: “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.”
Speculation that Mr Trump could be snubbed for Prince Harry and Meghan Markle’s May wedding in Windsor has been rife since the couple announced their engagement.
Ms Markle has previously described the President as “divisive” and a “misogynist”, while Prince Harry is good friends with Mr Trump’s predecessor Barack Obama.
The Royal wedding claim is the latest in a string of extraordinary disclosures from Mr Wolff, who has been dismissed by Mr Trump as a “fraud” peddling a work of “fiction”.
In the Mail on Sunday interview, Mr Wolff also suggested the President did not know what Brexit was until two weeks before the 2016 referendum.
And he raised the prospect of potentially awkward scenes if Mr Trump does come to Britain on his controversial proposed state visit, saying the Republican would “try to Trumpalise the Queen and Buckingham Palace” and that he saw the Queen “in reality TV show terms”.
Mr Wolff made fresh claims about the President’s mental state in the interview, claiming some White House officials wondered whether he had learning disabilities, Attention Deficit Hyperactivity Disorder, or even dementia.
“They discuss it at the White House: his apparent inability to read one page or one paragraph. He can’t even follow a PowerPoint,” he told the newspaper.
“Everyone around him says the symptoms have got worse in the year he has been in office – his attention span has lessened, his verbal patterns are more peculiar.”
The President has dismissed allegations he is unfit for the Oval Office and in a Twitter tirade on Saturday proclaimed himself to be a “very stable genius”.
On Sunday, he continued to attack Mr Wolff, describing his tome as a “Fake Book” from a “totally discredited author”.
It comes as British Prime Minister Theresa May brushed off concerns about Mr Trump’s mental state.
Mrs May said that in her dealings with him she had always found the President determined to act in the best interests of the US.
On the subject of a visit to the UK by Mr Trump, she told the BBC’s Andrew Marr Show: “He will be coming to this country.”
The PM did not say whether it would be a full state visit or simply a working trip to mark the opening of the new US embassy in London, which is expected to happen in February.