Boris Johnson's race row aide quits

James McGrath, the Mayor's deputy chief-of-staff, claimed his remarks, which appeared on the internet, had been taken out of context.

But Mr Johnson decided that his political strategist, who played a key role in his election, would only provide "ammunition" to critics if he stayed in his post.

City Hall insiders said Mr Johnson understood that the remarks could be damaging to his attempts to be seen as a mayor for all Londoners. One said: "Boris is not a racist and he's damned if he's going to be portrayed as one by a careless remark."

In an interview with journalist Marc Wadsworth, posted on a "citizen journalism" website, Mr McGrath was challenged with the suggestion by the writer Darcus Howe that Mr Johnson's election cou ld cause an exodus of Caribbean immigrants.

He responded: "Well, let them go if they don't like it here." The 34-year-old Australian later posted a comment on the website, thelatest.com, in which he said: "The columnist suggested that older people from the Caribbean might migrate back to their homelands if Boris Johnson became Mayor.

"I felt that this suggestion was ridiculous and intended as a slur and responded by saying with words to effect of 'let people go if they don't like it here'. To imply that I meant that all black people who didn't support Boris Johnson should leave the country is utterly absurd and incorrect. And I would ask please that this insinuation is immediately retracted."

The Mayor defended him against allegations of racism but had already decided that he would have to go.

In a statement, Mr Johnson said: "It is with great regret that I have accepted the resignation of my political adviser, James McGrath. James is not a racist. 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 allowed doubts to be raised about that commitment.

"We both agree that he could not stay on as my political adviser without providing ammunition for those who wish to deliberately misrepresent our clear and unambiguous opposition to any racist tendencies."

Mr McGrath was appointed director of political strategy at City Hall following Mr Johnson's victory last month.

Mr Wadsworth, a respected commentator on black issues, said he was "shocked" at Mr McGrath's "hubris", adding: "I did not rush into print with a story straight after the extraordinary meeting with McGrath on 20 May."

"I attempted a private dialogue. But, for a month, despite reminders from me, I waited in vain for McGrath's promised response. I found his apparent contempt for the serious issues raised - both at the meeting and in a document I sent him afterwards at his request - very disrespectful."

During the election campaign it emerged that Mr Johnson had once described black children as "piccaninnies" in a newspaper article.

Ken Livingstone said yesterday: "The real culture of the Johnson administration is becoming clear - one totally at odds with the needs of London as the most diverse city in the world."

Influential Tory blogger Iain Dale criticised Mr Johnson for letting Mr McGrath go. He said: "All Boris has done is attempt to appease people who are not capable of being appeased. What he should have done is stand by the man who has stood by him through thick and thin. What he has done is create a rod for his own back."