Neil Sankey has spent his life investigating organised crimes. As a former British police officer with almost 20 years experience, he was seconded to elite units of Scotland Yard through most of the 1970s and now runs his own private detective agency in California.

Over the years he has been involved in some big investigations. As part of the Special Branch and Bomb Squad he monitored British leftwing groups and the IRA, and in America his clients have included several big car companies.

But never has he handled anything quite as monumental as the investigation that is absorbing his energies today.

Sankey is pursuing what he believes to be fraud on a gigantic scale — a conspiracy, no less, to infiltrate and destroy the free world by putting a foreign imposter into the White House.

Sankey is a member of the fringe alliance known widely as the Birthers (he dislikes the expression, considering it pejorative). Together with other activists, he seeks to prove that Barack Obama is not a true American and is therefore ineligible to be president.

Over the past year Sankey has been at the centre of some of the most aggressive efforts by the Birthers to unseat the president. At the end of last year he tried to block Obama's inauguration by contacting all 538 electoral college representatives who formally elect the president. More recently, he has carried out his own probe into Obama's personal identification history which has revealed, he believes, a suspicious multiplicity of social security numbers.

Sankey says his fascination began with the realisation "that this man wasn't what he said he was. He wasn't an ordinary Democrat — he was far more extreme than that." So about a year ago he began reading blogs and websites that claimed to expose Obama's foreign roots, his spurious Hawaiian birth certificate and the $2m White House cover-up that has prevented the public finding out about the plot.

His travels put him in touch with Orly Taitz, one of the most energetic and flamboyant of the Birther leaders. Of Moldovan extraction, she emigrated via Israel to California where she works as a dentist and lawyer. She has filed numerous legal suits around the country on behalf of serving US military personnel attempting to prevent their deployments to Iraq and Afghanistan on the grounds that they should not be taking orders from an illegally serving commander-in-chief.

Sankey's journey from having worked in some of the most elite police units in Britain to taking part in a movement dedicated to the pursuit of a paranoid conspiracy theory may seem bizarre. But he insists it has been a natural progression. He joined the Hampshire force in 1961, and was seconded as a detective sergeant to Scotland Yard where he developed a specialism tracking leftwing political groups and the IRA.

"We created an operation into what we called revolutionary criminality — monitoring leftwing bookshops and extremist literature, following the leftist fringe and the Marxist links of the IRA."

In 1980 he moved to California, set up his agency, and became a naturalised American in 1985.

Sankey contends that his police experience in England now informs his fight against Obama. "It's quite obvious to me — America is heading towards a socialised state just as has happened in Europe. Socialised medicine, everyone on the dole, and when everything collapses you tip the scales into Marxism."

He also believes his training in Scotland Yard is now reaping benefits for the Birthers. The same techniques he used to analyse the IRA's associations he is now applying to Obama. Most recently, he carried out an exhaustive search of databases that he claims threw up 140 different identification numbers and addresses for "Barack Obama". He admits the findings prove nothing — there is nothing to link the entries to the president — but he believes it raises further doubts that need investigating.

Taitz says Sankey's UK police expertise has been invaluable. "He has had superb training. I have the greatest respect for Scotland Yard."

The Birther movement is not a unique phenomenon within US politics. Bill Clinton was accused by conspiracy theorists of having murdered his friend and White House legal adviser Vince Foster; George Bush had to contend with the Truthers who believe he was the mastermind behind the 9/11 attacks.

But the Birthers are unlike previous movements in that they are focused on who Obama is rather than what he does.

"There is no other president who has had his citizenship questioned in this way," says Patricia Turner, an expert in folklore at the University of California, Davis. Turner says that the popular Birther theories that Obama has used fake Hawaiian documents to disguise the fact he was born in Kenya or Indonesia are retellings of an old story. "This is just a proxy for old-fashioned racism. They are driven by hostility towards anything they see as foreign or exotic."

Although the Birthers are on the fringe of American politics, they are part of a wider surge of rightwing anger towards Obama's perceived socialist policies that is sweeping the country.

As such they can command considerable support. An internet petition demanding an official inquiry into Obama's origins has been signed by almost 500,000; critics say the number is inflated by multiple clicks.

Like any virulent conspiracy theory, that of Obama's birth has proved immune to the intervention of fact. When Obama's birth certificate in Hawaii was digitally scanned for all to see, it was denounced as a forgery. The birth notices printed by two Hawaii newspapers announcing his birth in August 1961 were similarly dismissed.

Dozens of legal actions have been brought before the courts by Taitz and other Birther leaders, and so far every one has been thrown out. Last month a federal judge dismissed Taitz's lawsuit seeking to challenge the chain of military command up to Obama as commander-in-chief. In a devastating ruling, the judge accused Taitz of trying to "emasculate the military" in a way that would "leave this country defenceless".

None of these setbacks have dissuaded Sankey. He says accusations of racism are smears that he has come to expect. "The objection is not Obama's colour but his politics. I like him as a person, I just wish he was genuine."