A Republican volunteer who claimed a black man carved a “B” into her cheek after seeing a McCain bumper sticker on her car has confessed that she invented the story.

Pittsburgh police said that Ashley Todd, a white college student from Texas, is being charged with making a false report to police after she alleged that she was mutilated by a supporter of Barack Obama.

The 20-year-old initially told police that she was attacked on Wednesday night in Pittsburgh as she attempted to use a cash machine. She claimed that her assailant, a tall black male, grabbed her from behind, put a knife to her throat and forced her to hand over $60.

The canvasser told investigators that after the man noticed a John McCain sticker on her car, became angry and punched her in the back of the head, knocking her to the ground and telling her “you are going to be a Barack supporter,” police said.

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She said he continued to punch and kick her while threatening “to teach her a lesson for being a McCain supporter,” police said. She claimed he then sat on her chest, pinned her hands down with his knees and scratched a letter “B” into her face using what she believed to be a knife.

The McCain campaign immediately denounced the attack as “sick and disgusting”, while Barack Obama was forced to issue a statement deploring such behaviour and demanding the assailant be swiftly brought to justice. Both Mr McCain and his running-mate Sarah Palin telephoned Ms Todd expressing their concern.

However police cast doubt upon her claims after video surveillance failed to show her at the cash machine. She was asked to take a polygraph test. As her story unravelled, she was eventually forced to admit that she had invented the incident.

It is not clear how she got the mark on her face but a photograph that has surfaced on the internet shows the “B” to be backwards, suggesting she may have cut it herself using a mirror.

The incident illustrates the depth of hatred of some McCain supporters towards the Democratic nominee, who would become America’s first black president if he wins the November 4 election.

Race has been a sensitive issue in the contest and a number of prominent Republicans, including former Secretary of State Colin Powell, have criticised the increasingly divisive tone of the McCain-Palin campaign. Some supporters have been freely expressing their distaste for a black president to reporters attending McCain rallies, while cries of "terrorist!" and "kill him" have been heard from the crowd at televised events.

On Thursday, Fox News Executive Vice-President John Moody, who is a Pittsburgh native, wrote on his blog that the incident, if proven false, would irreversibly damage the McCain campaign.

"This incident could become a watershed event in the 11 days before the election," he wrote. "If the incident turns out to be a hoax, Senator McCain’s quest for the presidency is over, forever linked to race-baiting."

Neither Ms Todd nor her family were responding to calls for comment.

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