UKIP has accused Mr Obama of "parroting the Downing Street line"
British anti-EU campaigner Boris Johnson has hit back at the 'Remain' side, following US President Barack Obama's Brexit warning.
The London mayor, one of the most high-profile figures in the Vote Leave camp, said the rival group "think they have bombed us into submission".
But Mr Johnson suggested they "think it's game over", were "crowing too soon" and could be "ignoring the resilience and thoughtfulness of many middle-of-the-road" voters.
His remarks in his Daily Telegraph column come after the president said Britain may have to wait a decade for a free-trade deal with the US if it votes to quit the EU on June 23rd.
Mr Johnson claimed a previously undecided senior public servant had been so outraged at Mr Obama's earlier "back of the queue" remark that he had decided to vote to leave.
The potential next Tory leader also launched a fresh attack on the EU, saying the "euro crisis is far from over, and that the EU remains a gigantic engine of job destruction".
And he claimed the British Prime Minister David Cameron achieved "two-thirds of diddly squat" in his negotiations on EU reform in the lead-up to the referendum.
UKIP leader and Leave campaigner Nigel Farage has accused Mr Obama of "parroting the Downing Street line" on why the UK should remain in the EU.
Another Out campaigner, Michael Gove, has warned Britain will be subject to a migration "free-for-all" if it stays in the EU, with millions more people from countries including Turkey and Albania set to join an expanded union in the future.
The UK Justice Secretary also told The Times that the NHS will face "unquantifiable strain" if Britain remains an EU member.
Mr Johnson has been widely criticised for suggesting that Mr Obama's Kenyan heritage might have given him negative feelings towards Britain.
But Labour former frontbencher Chuka Umunna said: "What Boris Johnson said was disgraceful. For any Londoner, for our Mayor of London to raise the issue of the President of the United States' ethnicity and then very publicly have to be slapped down by that President for doing so...it's an embarrassment".
"This man is simply not fit to hold the office that he clearly aspires to, which is the Prime Minister of our country".
Mr Johnson, whose article contained a reference to Mr Obama allegedly removing a bust of Winston Churchill from the White House, wrote in Friday's The Sun: "Some said it was a symbol of the part-Kenyan president's ancestral dislike of the British empire - of which Churchill had been such a fervent defender".
But speaking at the UK Foreign Office alongside Mr Cameron, Mr Obama made clear his admiration for the wartime leader.
"Right outside the door of the Treaty Room, so that I see it every day - including on weekends when I'm going into that office to watch a basketball game - the primary image I see is a bust of Winston Churchill. I love the guy," he said.