The conspiracy theory about President Obama that refuses to die

They are known as the ''birthers'': a group of lawyers, conservatives - even some members of Congress - who refuse to accept that the President, Barack Obama, was born in the United States.

Even though the Obama campaign team published an official certificate from the state of Hawaii stating that he was born there on 4 August, 1961, and two Hawaiian newspapers confirmed they had published birth notices at the time, a significant portion of the US population - 11 per cent, according to one poll - still dispute Mr Obama's birth details.

Born identity . . . a fake birth certificate puts Barack Obama's birthplace on Mombassa, Kenya.

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

But despite a body of evidence that confirms that Mr Obama is an American - born and bred - a claim that has been checked by independent fact-checking organisations such as the University of Pennsylvania's FactCheck.org - the conspiracy theories continue.

It reached new heights this week when the website WorldNetDaily said it had obtained a copy of a Kenyan birth certificate for Mr Obama. The right-wing website carried images of the certificate - allegedly issued by the Republic of Kenya on February 27, 1964, that says a Barack Hussein Obama was born in a hospital in Mombassa, Kenya, on August 4, 1961.

The ''smoking gun'' document had been obtained by Orly Taitz, an lawyer from California who filed an unsuccessful lawsuit in the Supreme Court querying Obama's birth. She also attended a law conference at which the Chief Justice, John Roberts, was speaking to urge him to consider the issue. She was escorted out by security guards.

The document soon brought howls from the liberal blogosphere. The website Daily Kos said Kenya, while independent in December 1963, did not declare itself a republic until a year later.

The hospital named on the birth certificate has no record of the birth, according to research by Daily Kos. Now even WorldNetDaily acknowledges that the Taitz document ''is probably not authentic'', according to its ''investigative operatives in Africa''.

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 printout of the record, which bears the state seal.

But several journalists have called the state's health director, Dr Chiyome Fukino, seeking further clarification, and he has said: '' I have seen the original vital records maintained on file by the Hawaii state department of health verifying Barack Hussein Obama was born in Hawaii and is a natural-born American citizen.''

The birther story keeps leaking into the mainstream. The CNN presenter Lou Dobbs says the allegations are false, but he keeps hosting the birthers on his prime-time program; Sean Hannity on Fox has featured Andy Martin, a prominent birther, and the radio shock jock Rush Limbaugh told his listeners that Mr Obama ''has yet to have to prove that he's a citizen''.

The matter has even taken hold with some Republican legislators. Representative Bill Posey of Florida has introduced a bill into the House of Representatives that would require a presidential candidate to produce a birth certificate together with ''any such documentation as may be necessary'', and proposed the bill take effect before 2012, in time to require Mr Obama to produce the goods.