David Plouffe, Obama’s campaign manager, described it as “the most shameful, offensive fear-mongering we’ve seen from either party in this election”. Obama has spent much of the campaign emphasising he is a Christian not a Muslim and did not study at a madrasa.

Aides for Clinton, who is fighting a last-ditch battle to keep her hopes of the White House alive, initially tried to brush off the furore, but later denied having anything to do with the distribution of the picture.

“I just want to make it very clear that we were not aware of it, the campaign didn’t sanction it and don’t know anything about it,” Clinton spokesman Howard Wolfson told reporters. “None of us have seen the email in question.”

Obama and Clinton next go to the polls in the Texas and Ohio primaries next week. If Clinton loses either, her bid for the Democratic nomination could be over.

The picture showing Obama in a turban during a visit to Kenya in 2006 first appeared on the Drudge Report website on Monday. The site said it was circulated by Clinton’s staffers and quoted one saying: “Wouldn’t we be seeing this on the cover of every magazine if it were (Clinton)?”

The picture was taken when Obama went on a visit to Africa as a senator. Obama, whose father was Kenyan, visited Wajir in Kenya’s north-east, close to the Somali and Ethiopian borders, and was dressed by locals as a Somali elder.

The Clinton team hit back at the criticism, saying Obama’s team was trying to distract attention from a foreign policy speech she gave on Monday.

Marijuana, cocaine

The Obama campaign has repeatedly claimed it has been the target of dirty tricks by Clinton’s team. A senior member of her staff, Bill Shaheen, had to resign last year after raising Obama’s admitted use of marijuana and cocaine as a youth. A junior staffer resigned in December after forwarding an email suggesting Obama is a Muslim.

Plouffe described circulation of the picture as part of “a disturbing pattern”. “It’s exactly the kind of divisive politics that turns away Americans of all parties,”he said.

His comments suggest that the cordial relationship that existed between Obama and Clinton in last Thursday's televised debate in Texas was unlikely to carry into Tuesday night’s debate in Cleveland, Ohio.