THEY are known as the ''birthers'' - a group of lawyers, conservatives, even some congressmen, who refuse to accept that President Barack Obama was born in the US.

Despite the fact that his campaign published an official certificate from the state of Hawaii stating he was born there on August 4, 1961, and two Hawaiian newspapers confirmed they had published birth announcements, a significant portion of the US population - 11 per cent - still dispute his birth details.

The issue of where Mr Obama was born is germane because the US constitution requires the president to be a ''natural-born citizen of the United States''.

Despite a body of evidence confirming that he is American-born and bred, the conspiracy theory continues to gather steam.

The ''Obama is a foreigner'' myth now threatens to supplant the ''Obama is a secret Muslim'' story, which has been dented by reports and footage of the family attending church.

The birther story reached new heights this week when the website WorldNet Daily claimed to have a copy of Mr Obama's Kenyan birth certificate. The right-leaning website carried images of a relatively simple document allegedly issued by the Republic of Kenya on February 27, 1964, that says a Barack Hussein Obama had been born in a hospital in Mombasa, Kenya, on August 4.

Writer Jerome Corsi, a prominent birther, said on the site that the certificate ''bears none of the obvious traits of a hoax''.

The document was obtained by Orly Taitz, a California attorney who filed an unsuccessful Supreme Court lawsuit querying Mr Obama's birth.

The latest documents soon brought howls from the liberal blogosphere. Daily Kos, one of the most vigorous websites trying to combat the birther claims, said Kenya, while independent in 1963, did not declare itself a republic until a year later.

The hospital named on the birth certificate has no record of the birth either, according to research by the Daily Kos.

Part of the problem for Mr Obama is that he has not produced his original birth certificate. Hawaii digitised all its original records some years ago and now gives people a print-out, which bears the state seal.

The state's Health Director, Dr Chiyome Fukino, has confirmed that he had seen the original records verifying that Mr Obama was born in Hawaii.

That's still not enough for some. A Research 2000 poll, sponsored by Daily Kos, found that not only were 11 per cent of Americans convinced Mr Obama was not born in the US, over half of Republicans were doubtful.

Some 28 per cent of Republicans said they did not believe Mr Obama was born in the US; 30 per cent were not sure and only 42 per cent said yes, the President was born in America.