John Reid, the Home Secretary, appealed for calm today over the discovery of radiation at several locations associated with the death last week of an exiled former KGB officer.

In an emergency Commons statement on the death last Thursday of Alexander Litvinenko, Mr Reid said the nature of the radiation emitted by the substance polonium-210 was such "that it does not travel over long distances - a few centimetres at most.

"There is no need for public alarm over the fact that radiation may have been found, since there is such a short distance encompassed in its effect," Mr Reid told MPs.

Earlier today, the Health Protection Agency (HPA) said that three people who contacted an NHS helpline after news that Litvinenko had been poisoned by a radioactive substance had been referred for tests because they showed possible symptoms of radiation poisoning.

Doctors want to make sure that the three have not been contaminated by polonium-210, a significant amount of which was discovered in Litvinenko's urine hours before his death.

Traces of polonium have since been found at Litvinenko's North London home, as well as at a London sushi bar and a Mayfair hotel that he visited on the day that he was poisoned on November 1. Sky News reported today that further traces had been found at two other Central London addresses.

An HPA spokeswoman said that more than 450 people who could have come into contact with Litvinenko, or otherwise feared contamination, had contacted a hotline set up after his death and 18 of them had been passed on to the HPA for follow-up.

Of those 18, three had displayed symptoms "that could potentially have been caused by radiation exposure", she added. "We need to rule them out, or rule them in, as soon as possible."

The full results of the tests could take up to a week.

In his Commons statement, Mr Reid described the tests as a "very precautionary measure".

The Home Secretary said that the Russian authorities had been asked to provide "all necessary co-operation" as the police investigation proceeded but cautioned MPs against reaching early conclusions. He said the police had ruled nothing in or out in the course of their investigations so far.

In a statement dictated and signed on his deathbed, Litvinenko, 43, pinned the blamed for his death on President Vladimir Putin, whom he accused of being "barbaric". The Kremlin has denied any involvement.

Earlier today, ministers and top security on the so-called Cobra committee, chaired by Mr Reid, held another emergency meeting to discuss the case and its potential diplomatic fallout.

The case is being investigated by Scotland Yard's counter-terrorist command, which has asked Moscow to pass on any information it has about Litvinenko's death.

The Scotland Yard team was due today to receive a dossier drawn up by Litvinenko on the Kremlin’s takeover of the world’s richest energy company, Yukos.

It emerged yesterday that Mr Litvinenko travelled to Israel just weeks before he died to hand over evidence to a Russian billionaire of how agents working for President Putin dealt with his enemies running the oil company.

He passed this information to Leonid Nevzlin, the former second-in-command of Yukos, who fled to Tel Aviv in fear for his life after the Kremlin seized and then sold off the £21 billion company.

Mr Nevzlin told The Times that it was his "duty" to pass on the file. "Alexander had information on crimes committed with the Russian Government’s direct participation," he said."He only recently gave me and my attorneys documents that shed light on the most significant aspects of the Yukos affair."

Investigators have told The Times that Mr Litvinenko had apparently uncovered "startling" new material about the Yukos affair and what happened to those opposing the forced break-up of the company.