There may have been multiple attempts to kill Russian ex-spy Alexander Litvinenko before he died, BBC One's Panorama programme has discovered.
An attempt may have been made on Mr Litvinenko's life in October
The first poison bid may have come two weeks before he met Mario Scaramella in a sushi bar on 1 November.
It may have been at the same restaurant, but when Mr Litvinenko met former KGB men Andrei Lugovoi and Dimitri Kovtun on 16 October.
Mr Litvinenko suffered a fatal overdose of radioactive isotope polonium-210.
Traces of the poison were discovered not where Mr Litvinenko and Mr Scaramella sat, but elsewhere in the restaurant, most likely where Mr Litvinenko met the Russians.
Several other sites, including a hotel visited by Mr Lugovi and Mr Kovtun, were also contaminated.
Mr Scaramella, an academic, said he had no idea how polonium reached the sushi bar and denied "absolutely" taking part in the poisoning.
"I know they closed it because they found the polonium, but seems it was not in the place where we seated. So lots of things must be clarified. Where we seated there is no polonium," he said.
It was widely reported that Mr Scaramella had tested positive for polonium.
But Panorama has discovered that his initial test results were inaccurate. Subsequent tests proved negative.
Mr Litvinenko had collaborated with Mr Scaramella on an Italian KGB mole-hunt.
"Some of this information was lethal information, shall we say. Other people have been killed for this kind of co-operation," Mr Scaramella said.
Mario Scaramella tested negative for polonium, Panorama has learned.
Mr Litvinenko's widow Marina said her husband's tea may have been spiked at another meeting with the two ex-KGB men on 1 November.
She said: "At the Millennium Hotel Sasha (Mr Litvinenko) told me he met Lugovoi and during this meeting he had drunk tea.
"He said it was tea already served, on the table, and he just took this cup of tea, and he didn't finish it at all, and how he later said tea wasn't very tasty, 'because it was cold'."
Panorama found Mikhail Trepashkin, a jailed former officer with the Russian secret service, who was ordered to monitor Mr Litvinenko in 2001.
The Kremlin has called Marina Litvinenko a liar
The programme also visited Laboratory Number 12 in Moscow.
An anonymous ex-Soviet intelligence officer said: "It's the laboratory that every year gets its budget to work with radioactive poisons."
Prodi 'a KGB friend'
Panorama has also obtained a document classified "top secret" in Italy in which Mr Litvinenko accuses Italian prime minister Romano Prodi of being a friend of the KGB.
Mr Scaramella said: "Some qualified sources, including Litvinenko, told me that some officers in Moscow considered him as their man, KGB man."
Mr Litvinenko was warned off defecting to Italy "because there are some big friends of Russia in this country", he added.
Mr Prodi has always denied having KGB links.
Marina Litvinenko told Panorama the poisoning could not have been carried out without Russian President Vladimir Putin's knowledge, as he is "behind everything that happens in Russia".
Mr Putin's spokesman, Dimitry Peskov, responded: "I answer directly that Russia has not done it and it is absurd even to think about it.
"If she says that Russia has killed Sasha, she's a liar for these words."
Panorama: How to Poison a Spy will be shown on BBC One at 2030 GMT on Monday 22 January.