Washington: President Donald Trump phoned into a radio show in London on Thursday to offer his thoughts on the upcoming British election, hinting at advice he had for British Prime Minister Boris Johnson on the still-ongoing negotiations to leave the European Union.

"He's in a very difficult position, and I think he's willing to do what no one would do," Trump said of Johnson, who was forced earlier this month to ask EU leaders for an extension on the Brexit deadline, originally set for Thursday.

But the president also criticised the withdrawal agreement that Johnson reached with EU leaders in Brussels, suggesting it would hinder plans for a bilateral trade deal between the US and Britain.

"To be honest with you, this deal, under certain aspects of the deal, you can't do it," Trump said.