Traces of the radioactive substance have since been discovered on two British Airways flights
What is polonium-210?
It is a naturally occurring radioactive material that emits highly hazardous alpha (positively charged) particles.
It was first discovered by Marie Curie at the end of the 19th century.
There are very small amounts of polonium-210 in the soil and in the atmosphere, and everyone has a small amount of it in their body.
But at high doses, it damages tissues and organs.
However the substance, historically called radium F, is very hard for doctors to identify.
Philip Walker, professor of physics, University of Surrey said: "This seems to have been a substance carefully chosen for its ability to be hard to detect in a person who has ingested it."
What is the risk to other people from the dose Mr Litvinenko received?
It cannot pass through the skin, and must be ingested or inhaled into the body to cause damage.
And because the radiation has a very short range, it only harms nearby tissue.
However, there is a theoretical risk that anyone who came into contact with the urine, faeces, and possibly even sweat, of Mr Litvinenko could ingest a small amount of the polonium.
William Gelletly, professor of physics at the University of Surrey, said: "Polonium-210 is very unlikely to have contaminated any staff who treated Mr Litvinenko or anyone who came in contact with him since they would have had to ingest or breathe in the contaminated fluids from his body."
Where does polonium-210 usually occur?
It has industrial uses such as static control and as a heat source for satellite power supplies, but is not available in these areas in a form conducive to easy poisoning.
It is also present in tobacco.
Professor Dudley Goodhead, Medical Research Council Radiation and Genome Stability Unit, said: "To poison someone much larger amounts are required and this would have to be man-made, perhaps from particle accelerator or a nuclear reactor."
Where would someone obtain polonium 210 from?
Although it occurs naturally in the environment, acquiring enough of it to kill would require individuals with expertise and connections.
It would also need sophisticated lab facilities - and access to a nuclear reactor.
Alternatively, it could have been obtained from a commercial supplier.
Polonium 210 can either be extracted from rocks containing radioactive uranium or separated chemically from the substance radium-226.
Production of polonium from radium-226 would need sophisticated lab facilities because the latter substance produces dangerous levels of penetrating radiation.
How could polonium 210 have got onto the BA flights?
It is not clear whether the polonium was carried on to the flight in its raw form in some type of container, such as a vial, or whether the source of the radiation was a passenger who had ingested the polonium before boarding the flight.
Jon Miles, group leader for airborne radionuclides at the Health Protection Agency, said both ways were feasible.
He said: "It is possible if it was in a vial that was not well sealed it could have got out. It is a material that does spread very easily."
If ingested, polonium 210 comes out in sweat, vomit, urine and faeces. Mr Miles said that if these contaminated fluids were on a person's clothes they could rub off.
He said the health advice for the flight passengers would be similar to that for the people who had dined at the West End sushi restaurant where Mr Litvinenko had dined.
"Those actually there at the time will probably need to get in touch for more detailed analysis. But those on a later flight on the same plane might not have to because the risk is much lower."
Mr Alexander Litvinenko, reputed to have been an Ex-Russian spy who had defected to Britain, died last night in mysterious circumstances. He had alleged many associations between people in high places and organised international crime, implying that President Putin and Romano Prodi among others had been involved personally. He had sought political asylum in UK in 2000 and became a naturalised citizen in October this year, just weeks before his death.

Mr Litvinenko was said to have been investigating the shooting of Russian journalist and human rights activist Anna Politkovskaya, a well-known critic of Russian activities in , in her apartment in Moscow October 7, 2006. It is reported that he had met two Russians in a hotel room, one said to be former member of the  in connection with the affair and he went on to meet Mario Scaramella at a  bar in Piccadilly where some papers were exchanged. Some hours after this he was taken ill. 

Mr Litvinenko was admitted to , north London on November 1, 2006 complaining of feeling sick. By November 11, he was said to be suffering from ''serious poisoning''. A week later he was transferred to  in central London. A week later he was said to have been poisoned by ingesting , once used in rat poison, but, in the opinion of some doctors, there were signs of , including loss of weight and shedding of hair. Various explanations of his condition were offered. Last night he suffered a heart attack, after having left a message blaming President Putin for having him killed. 

The radioactive  -210 was found in his blood and urine as reported by the . The post-mortem was cancelled. Subsequently the Agency examined conditions in the hospitals in which Mr Litvinenko has been treated. Police visited the Itsu sushi restaurant in Piccadilly, his home in Muswell Hill and the Millennium Hotel, Grosvenor Square where the meeting on November 1 had been held. Traces of radiocactivity were found in all three places. It is speculated that the polonium was probably eaten by Litvinenko as a substance that could be combined with a salt-like substance, such as .

It was reported that a meeting had been held in Cabinet Office Briefing Room A (COBRA) used for high level emergency planning and control, to consider the implications of these events. The Foreign Office asked Moscow for a response to the accusation of Russian involvement and President Putin himself dismissed the allegations saying (before the cause of death had been established) that there was no proof of an unnatural death and that the case was being used as a "political provocation".