A senior Australian adviser to London Mayor Boris Johnson has been forced to resign in a race row after he made disparaging remarks about people from the Caribbean.

James McGrath, 34, quit after suggesting older African-Caribbean people should return to the West Indies if they did not like the new mayor.

Asked in a tape-recorded interview if Johnson's mayoral victory would trigger an exodus of immigrants from the UK to the Caribbean, he replied: "Well, let them go if they don't like it here."

McGrath, the mayor's deputy chief of staff who hails from the north Australian state of Queensland, made the comment in a meeting last month with Marc Wadsworth, a black activist and London-based journalist.

It is the latest race controversy to hit the new mayor and commentators said the row highlights how sensitive the issue is for Johnson.

After initially defending him, Johnson said McGrath, a Tory party adviser for almost a decade, was resigning to avoid "providing ammunition" to critics because the comments were "taken out of context and distorted".

Hours after his comments appeared on the Internet news site www.the-latest.com at the weekend, Johnson announced his senior advisor, who played a key role in his win over Labour's Ken Livingstone last month, had quit.

"It is with great regret . that I have accepted the resignation of my political adviser, James McGrath," Johnson said in a statement. "Unfortunately, his remarks . made it impossible for him to continue in that role.

"James is not a racist. I know that. He shares my passionate belief that racism is vile, repulsive and has no place in modern Britain. But his response to a silly and hostile suggestion put to him . allowed doubts to be raised about that commitment."

Conservative Leader David Cameron, who was consulted over the decision, told reporters on Monday: "I don't think he (McGrath) is racist, no, but I think it was a lapse in judgment.

"It was regrettable, it was something he shouldn't have said," said Cameron, who worked with McGrath in a previous role.

Dawn Butler, Labour MP for Brent South, said the comments were "disrespectful". "A lot of people who supported Boris Johnson will be disappointed," she told The Guardian.

During the mayoral campaign, Johnson was criticised over a Daily Telegraph article where he referred to black children as "piccaninnies".

Last week he was under fire from equality campaigners after he decided that "Rise", one of London's biggest free music festivals, should drop a central anti-racist message.