BANGKOK, Thailand — Thailand’s prime minister has never tried to conceal his English roots. Born in Newcastle, educated at Oxford, Abhisit Vejjajiva is promoted as lifting Thailand’s esteem with his refined background.

But some of Abhisit’s political opponents allege his British roots run a little too deep.

In claims reminiscent of America’s “birthers” — who insist Barack Obama can’t prove his birth on U.S. soil — the Thai premier is accused of concealing British citizenship.

This is more than an attempt to malign a man already accused of lacking a common touch with Thailand’s rural poor. If Abhisit is British, a claim he denies, attorneys hired by his wealthiest political opponent claim he can be tried for “crimes against humanity” in International Criminal Courts.

The U.K. has ratified a treaty subjecting its citizens to the war crimes tribunals, held in The Hague, Netherlands. Thailand has not.

“This man could drown in the blood that’s on his hands,” said Robert Amsterdam, a D.C.-based attorney for former Thai Premier Thaksin Shinawatra, at a recent press conference in Japan. “He has British citizenship. It will be up to Abhisit to show the prosecutor he has renounced.”

The citizenship claims, and the attempt to try Abhisit for war crimes, are the latest gambit by Thaksin to force the Thai premier from power — or at least publicly denigrate him.

Thaksin was ousted in 2006 by the Thai generals, who maintain far cozier relations with the current ruling party. The alleged “crimes against humanity” stem from last year’s violent crackdown on the Red Shirts, a faction of anti-government demonstrators.

Their Bangkok rallies, which peaked at about 200,000 people, were supported by Thaksin and designed to topple the so-called “Thai aristocracy” led by Abhisit.

After occupying parts of Bangkok for weeks, the rallies ended with guns-blazing Thai army raids. More than 90 were left dead, most of them protesters, some of them troops and Western journalists. Nearly 2,000 were injured and an arson spree left black smoke pluming above the skyline.

Amsterdam, Thaksin’s chief attorney, has sent the International Criminal Court a nearly 300-page entreaty to investigate Abhisit’s alleged wrongs against the protesters.

The document is a mix of credible witness reports, analysis from paid experts and outlandish claims supported only by anonymous sources.

Independent human rights groups and Western media outlets have reported that Thai forces appear to have fired on unarmed protesters, a claim the army denies. The government has also faced repeated criticism for failing to investigate any military wrongdoing during the clash.

But the Thaksin-financed report goes much further.

From the government’s perspective, live-fire offensives were justified after armed radicals had infested rally sites. Indeed, during one nighttime army raid, a prominent colonel and other soldiers were killed by black-clad men who defended the protest camp with assault rifles.

But Amsterdam’s report concludes these “men in black” were secretly “some of the best-trained snipers and marksmen drawn from all branches of the military.” It further alleges that soldiers posing as ambulance drivers “shuttled bodies to two colluding Bangkok hospitals, where the incriminating evidence was to be destroyed by cremation.”

These claims are not echoed by any independent rights groups and are attributed only to unnamed military sources.

Convicting Abhisit on anonymous claims, or dragging him into a setting better known for prosecuting Congolese militia chiefs, will be almost impossible.

But Abhisit and his party have responded to allegations that the premier is secretly British.

This claim “tramples on the hearts of Thai people,” said Attaporn Ponlabut, a parliamentarian with the Democrats, in an interview with Thai-language newspaper Matichon. “This is too much for the Thai people to endure.”

Abhisit denies holding U.K. citizenship and claims, through an aide, that he renounced it as a young man studying in England. When traveling to the U.K., Abhisit requests a visa just like any other Thai, he said.

The British government, however, would not confirm the renunciation when contacted by GlobalPost. “We don’t comment on individual cases,” said Sam Eversden with the U.K. Home Office. “Even if it’s the prime minister of Thailand.”

Abhisit was undoubtedly a U.K. citizen at one point. The prime minister was born in Newcastle, England, in 1964 to Thai parents. Anyone born on British soil before 1983 was automatically conferred U.K. citizenship by law.

The premier’s name at birth was “Mark A. Vejjajiva,” according to a U.K. birth registry obtained by GlobalPost through an online records agency. He also holds Thai citizenship, as does any person born to two parents with Thai citizenship regardless of location.

“He has the right to citizenship in both countries: one obtained from blood, one obtained from being born on the land,” said Bongkot Napaumporn from the Bangkok Legal Clinic at Thailand’s Thammasat University. “There is no law forbidding double citizens.”

Though any proof of U.K. citizenship would scar Abhisit politically, it probably wouldn’t terminate his premiership.

There is no Thai law forcing politicians to renounce their second citizenship before assuming power, Bongkot said. However, a second citizenship can be revoked if Thai authorities determine it jeopardizes national security, she said.

Thaksin’s lawyers contend the burden of proof lies with Abhisit. They’ve demanded he reveal documents proving his U.K. citizenship was revoked.

Thaksin’s citizenship status has also been scrutinized. Though he retains Thai citizenship, his Thai passport has been invalidated by the government. He has since acquired citizenship in Montenegro. The Thai government has also accused him of procuring five other passports and traveling under the name “Takki Shinegra.”

Though still influential among working-class Thais, the twice-elected former prime minister has not returned to Thailand for fear of a two-year prison sentence for fraud and terrorism charges stemming from last year’s protests.