Russian spy Alexander Litvinenko, whose tea was laced with polonium at a London hotel, may have been poisoned "not once but twice" with the substance, a public inquiry has heard.
Mr Litvinenko, 43, died in hospital nearly three weeks after he had consumed tea laced with the substance on November 1 at the Millennium Hotel in Grosvenor Square.
As the inquiry into his death got under way it heard that evidence suggested that Mr Litvinenko was first poisoned during a previous meeting at a security company in October that year.
Former KGB bodyguard Andrei Lugovoi and Dmitri Kovtun both deny any involvement and remain in Russia.
Counsel to the inquiry Robin Tam QC said: "One of the most significant things the evidence suggests is that Mr Litvinenko was poisoned with polonium not once but twice.
"It suggests two things - attempts to poison Mr Litvinenko were made at both meetings and that those attempts met with some success on both occasions."
Mr Tam explained that Mr Litvinenko had recalled feeling unwell at around the time of the first meeting in mid-October.
"Mr Litvinenko recalled vomiting on one occasion about two or three weeks before being hospitalised," Mr Tam said.
"Hair samples that are available indicate that Mr Litvinenko may well have been poisoned twice and that the first occasion being much less severe than the second."
The inquiry was also told that traces of "alpha radiation" had been found in Mr Lugovoi's hotel room following the final meeting at the Millennium, but there was none on buses that Mr Litvinenko had used to get there.
Thousands of Britons and tourists were put at risk from the radioactivity emitted by the substance.
Mr Tam said a public health alert was issued around the time of Mr Litvinenko's death when traces of polonium were found in "large numbers of places across London".
"Many thousands of members of the public, including British residents and visitors from overseas, might have been at risk from radioactivity," Mr Tam said.
The QC said many theories have been put forward about what happened to Mr Litvinenko, including that he was killed but also that he accidentally poisoned himself when handling the radioactive substance as part of a smuggling deal.
It has also been suggested that Mr Litvinenko committed suicide, Mr Tam said.
The counsel to the inquiry recounted the process that has unfolded in the eight years since Mr Litvinenko died, which saw an inquest replaced with this public inquiry.
Chairman Sir Robert Owen said that Mr Lugovoi and Mr Kovtun had been invited to give evidence via video link and he hoped they would accept.
Court 73, where the 10-week inquiry is taking place, was read a written statement in which Mr Lugovoi denied killing Mr Litvinenko.