The British government has given up hope of bringing the killer of Alexander Litvinenko to justice and is now concentrating its efforts on trying to ensure that similar murders do not take place in this country in the future, according to a senior source.
The former KGB officer Andrei Lugovoi, wanted by the British authorities for the poisoning by polonium of Mr Litvinenko – also once a KGB and later FSB officer– is very unlikely to be extradited, said the official, especially as he is now a member of parliament in Moscow.
The source said: "It is most unlikely that we will be able to get Lugovoi, but we can try to make sure that we don't have another Litvinenko." To this end, the UK authorities took a series of punitive measures after the murder of 44-year-old Litvinenko, in London in November 2006, to "send a strong message that this kind of thing will not be tolerated".
Britain froze links with the Russian secret service FSB, the successor to the KGB, and the suspension stays in place. The UK authorities have said they are prepared to pass information on counter-terrorism to Russian intelligence agencies other than the FSB.
Mr Litvinenko died after drinking from a cup of green tea while with Mr Lugovoi and two other Russians at the Pine Bar of the Millennium Hotel in central London. The teapot was so irradiated that the bar and several employees suffered from contamination, including a pianist who drank from the teacup after it had gone through a dishwasher.
Asked about who may have ordered Mr Litvinenko's killing, the source said: "Litvinenko was a former member of the FSB and he became a very unpopular man. I assume as a result of that his death was either officially sanctioned or that some faction or other of the FSB tried to get him."
The source continued: "If he [Litvinenko] had drunk the whole cup of tea he would have died instantly. It is only because he sipped it that he lingered and polonium was detected."
The latest round of acrimonious tit-for-tat measures followed broadcasting of claims that MI5 believed the Russian state was involved in the killing of Mr Litvinenko. This was followed by the Russians accusing a British diplomat, Chris Bowers, the acting director of trade and investment at the Moscow embassy, of being a spy, a claim strongly denied by the UK government. The Russians have also forced the British Council in Moscow to stop operations, and interrogated members of staff.
Gordon Brown used his first meeting with President Dimitry Medvedev of Russia last month to present a list of British grievances, chief among them the failure to extradite Mr Lugovoi. There was also the complaint that Russian intelligence activity in the UK has significantly increased recently, with, it is claimed, up to 30 Russian agents working out of the embassy and the country's trade mission, forcing MI5 to divert attention from combating Islamist terrorism.
But the Russian ambassador to the UK, Yuri Fedotov, maintained that Mr Lugovoi could not possibly have a fair trial in the UK "because of the political and emotional context" of the case. He also pointed out that Britain has refused to co-operate with the extradition of about 20 people wanted by the Russians, including the exiled billionaire Boris Berezovsky who had called for the overthrow of Vladimir Putin, when Mr Putin was president.
The UK source said that despite difficulties, relations between Britain and Russia were thawing and there was scope for co-operation in several fields.