Doctors are conducting radiological tests on three people after a former Russian spy was poisoned to death with a radioactive substance in London, British health officials said.

More than 450 people have reportedly called a hotline for health advice after Alexander Litvinenko, a former KGB colonel and vocal critic of the Kremlin, died last week.

His body was found to contain traces of polonium-210 — a highly radioactive isotope said to be about 250 billion timesmore toxic than cyanide, weight for weight.

Traces were also detected at his home and a hotel he visited, prompting health officials to urge any members of the public who may have frequented the same places to undergo testing.

So far, 18 people have been directed to Britain's Health Protection Agency.

"Of those 18, three have been referred as a precaution to a special clinic for radiological assessment," a British health official confirmed, declining to comment on their symptoms.

The remaining 15 will undergo standard tests such as urine analysis, she said.

The ex-spy fell ill in early November and told police he believed he was poisoned Nov. 1 while dining at a sushi bar, where he met a contact during his investigation into the slaying of Russian journalist Anna Politkovskaya.

Like Litvinenko, the murdered reporter had been critical of Russian President Vladimir Putin's government.

Implicated Putinin his murder

Within three weeks, Litvinenko's hair fell out, his throat became swollen, and his immune and nervous systems suffered severe damage.

He died last week accusing Putin of orchestrating his murder, but London police are still investigating it as a "suspicious death" and have not ruled out the possibility Litvinenko may have poisoned himself.

In the meantime, the possible contamination of the public hasput pressure on the government to explain what it knows about Litvinenko's death, though British officials have avoided blaming Moscow.

The affair could overshadow negotiations over energy issues and Russia's co-operation on the international standoff over Iran's nuclear ambitions.

In the strongest comments levelled at Moscow since Litvinenko's death, cabinet minister Peter Hain accused Putin of presiding over "huge attacks on individual liberty and on democracy" and acknowledged that relations between London and Moscow were at a difficult stage.

With files from the Associated Press