Mr Cameron has insisted he will not comment further on the US election despite his earlier intervention while Mr Trump was a rank outsider in the race.
Mr Trump previously said he would be tempted by Brexit if he was a British voter but today went further in an interview with The Hollywood Reporter.
Asked whether he would back Brexit, Mr Trump appeared confused by the term before adding: ‘Oh yeah, I think they should leave.’
The tycoon also announced he would be visiting Britain later this month – the day after the referendum to open his new golf course in Scotland.