|Litvinenko was poisoned while drinking tea at a London restaurant in 2006 [AP]
Russia was following the suspected killers of former spy Alexander Litvinenko before he was poisoned in London in 2006, but had been told to stand down by British authorities, according to remarks in leaked US diplomatic cables.
"[Russian Special Presidential Representative Anatoly] Safonov claimed that Russian authorities in London had known about and followed individuals moving radioactive substances into the city but were told by the British that they were under control before the poisoning took place," the cable said.
Litvinenko, a former KGB agent turned Kremlin critic, died in hospital after being poisoned with the radioactive substance polonium-210 in a restaurant.
The leaked US cable, quoting a meeting in Paris between a Russian official and Henry Crumpton, US Ambassador-at-Large, was released by WikiLeaks and published on The Guardian's website on Saturday.
Litvinenko's murder soured relations between the UK and Russia. The former spy's associates and some espionage experts have said that the Kremlin orchestrated the assassination.
However, the leaked cable appeared to be a record of a Russian denial of any involvement.
"The implication was that the FOR [Russia] was not involved, although Safonov did not offer any further explanation," the writer of the memo, dated December 26, 2006, noted.
Safanov was also said to have expressed appreciation for recent co-operative efforts between the US and Russia and "he cited the recent events in London - specifically the murder of a former Russian spy by exposure to radioactive agents - as evidence of how great the threat remained and how much
more there was to do on the co-operative front".
Officials in the UK have charged Andrei Lugovoy, a former KGB bodyguard, with the murder and has sought his extradition. Russia has refused and Lugovoy, who was later elected to the Russian parliament, giving him immunity from prosecution, has denied any link to the death.
A previously leaked US cable suggested that a senior US diplomat believed Vladimir Putin, the Russian prime minister, probably knew about the plot to kill Litvinenko, given the leader's "attention to detail".