At the Millennium hotel in Grosvenor Square, Mayfair, two policemen were posted by the revolving entrance doors yesterday. Inside, the Pine Bar was closed and fenced off with a wooden screen. One guest said bedroom 441 was blocked off and guarded by police.

The radioactive polonium-210 isotope is extremely toxic and the way it affects victims makes it an ideal weapon for a stealthy assassination.

Found naturally in uranium ore and produced in larger quantities in nuclear reactors, it can kill in several ways.

Even without the radiation it is highly toxic — in a similar way to metals such as lead, mercury or cadmium. Unlike these elements, polonium-210 also emits large amounts of alpha radiation.

Alpha particles are undetectable by Geiger counters unless the equipment is used at close quarters and without any barrier, such as flesh or even paper. This explains why a Geiger counter check of Litvinenko in hospital failed to register the radiation.

Although they are not as penetrative as gamma rays, the particles disrupt cell structures, causing the development of cancerous tumours.

When administered in relatively “large” amounts — such as a gram or two — they can cause horrific radiation poisoning. This seems to have been what killed Litvinenko.

Even with a minute dose, half of the victims can expect to die within 30 days and this life expectancy decreases as the dose rises. Symptoms including vomiting begin to appear an hour or so after the victim is irradiated.

That might lead doctors, as they did in Litvinenko’s case, to suspect food poisoning.

This is followed by a “latent” phase and then a further decline, including the loss of all hair and a massive depletion in white blood cells.

Dr Ian Fairlie, a consultant on radioactivity, said: “Polonium-210 is very potent indeed and it would only have taken around half a teaspoon of it to cause this.”

After Litvinenko ingested or drank the substance, minute amounts would have been released from his body through urine, faeces, vomit and sweat, perhaps explaining why traces have been detected at the hotel.

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