U.S. President Donald Trump had a 20 minute phone call with Boris Johnson on Tuesday but the favourite to replace Theresa May as British prime minister declined a face-to-face meeting due to a prior commitment, ITV's political editor said.

Trump praised Johnson just before he arrived in Britain for this week's state visit, saying he thought the former London mayor and former British foreign secretary would do a "very good job" as the next prime minister.

ITV's Robert Peston, citing a source close to Johnson, said Trump had offered a one-to-one meeting. Johnson is due to attend an event with other leadership candidates later on Tuesday. The source described the phone call as friendly and productive.