An African advocacy group that wanted Britain to leave the EU has quit the Brexit campaign in protest at Boris Johnson's "very alarming" comments about Barack Obama's Kenyan ancestry, which it said risked scapegoating immigrants.
The Africans for Britain organisation had been held up by the Vote Leave campaign as evidence of the latter's broad backing from all parts of society in its application to be the official anti-EU campaign.
But Africans for Britain has now resigned from the official Brexit campaign, saying it could not tolerate the mayor of London's comments in The Sun that the US president has an "ancestral dislike of the British empire" because he is "part-Kenyan".
Johnson made the comments in response to Obama's calls for the UK to vote to remain in the EU in the referendum on 23 June.
Africans for Britain said it was now suspending all campaign activity amid concerns that the anti-EU campaign is being taken over by a "radical wing" who will encourage prejudice against immigrants in the UK.
The organisation confirmed to BuzzFeed News that the decision was a direct result of the mayor of London's words: "We took our responsibilities after his comments."
It added: "We are ... moving forward with non-partisanship in this campaign in which we urge both camps to conduct a dignified campaign."
In a statement Africans for Britain expanded on the reasoning, explaining that the group had deep concerns about the way the anti-EU campaign was being fought: "We fear a takeover of the campaign by the radical wing which is likely to scapegoat immigrants.
"We are not confident that as the campaign heats up, initial promises to focus its message on trade opportunities outside the EU and regaining democracy will prevail.
"We hope the campaign on both sides tries to stay factual and to avoid being divisive. We will continue to believe in Britain from a non-partisan standpoint."
The group had argued that Britain leaving the EU would encourage trade with African and Caribbean nations and make it easier for people from those countries to travel to the UK.
But Africans for Britain said it had been left disappointed by the negative tone of the campaign.
"We backed #Brexit and @voteleave which we formally endorsed, with the belief that the campaign would be fought with a positive message," the group said separately on Twitter.
"The comments issued by @MayorOfLondon and endorsed by @Nigel_Farage, the biggest figures of #Brexit are very alarming to us," it said in another tweet.
Jamila Nana, Africans for Britain's founder, went even further and posted a series of tweets on her personal account saying she "will not be wasting my time pounding the pavement" for the campaign. The lengthy message began with a reference to the treatment of "house negroes".