By Frank Gardner
Security correspondent, BBC News
One year after the agonising death from polonium poisoning of former KGB officer-turned-dissident Alexander Litvinenko, relations between Britain and Russia have gone from strained to rocky.
Litvinenko died in a London hospital in November 2006
Litvinenko was a British citizen (his citizenship came through shortly before he was poisoned) and his death in a London hospital was investigated with some urgency by detectives from Scotland Yard's Counter Terrorism Command.
For them, this was not just a case of some Moscow spat spilling over onto the streets of London, it was the deliberate, planned murder of a Briton on British soil using a lethal radioactive substance that potentially endangered the health of many people.
In January 2007 the Metropolitan Police handed the Crown Prosecution Service (CPS) a file that contained, among other things, the name of their chief suspect in the case: Andrei Lugovoi, another ex-KGB officer who had met Litvinenko for tea at the time he fell ill.
Mr Lugovoi, who is now poised to enter Russian politics, denied then and continues to deny any involvement in the murder.
Yet police detectives who followed the forensic trail of polonium-210 around London and Europe say they can see no other explanation of how Mr Lugovoi and his effects could have been so heavily contaminated with polonium, while he himself escaped poisoning.
Andrei Lugovoi has denied involvement in the murder
Polonium emits radioactive alpha particles which can be stopped by human skin or even a piece of paper, but which can be lethal if ingested.
Litvinenko is believed to have drunk a heavy dose disguised in a pot of tea.
The CPS digested the police file and then, in May, announced it had enough evidence to charge Mr Lugovoi with murder, and instructed its lawyers to request his extradition from Russia to Britain to stand trial.
That, said Russian officials, was not going to happen, as it was "against the Russian constitution".
How about a trial in Russia instead, they offered?
Out of the question, said the CPS, which feared for the safety of key witnesses like the poisoned Russian's widow, Marina Litvinenko, if she went to Moscow to testify.
So British officials huffed and puffed, but Russia would not budge. Then in July the UK Foreign Secretary, David Miliband, announced the expulsion of four Russian diplomats without revealing whether they were spies or not.
There were fears Litvinenko's widow would be in danger in Moscow
Russia responded by expelling four Britons from the embassy in Moscow.
Russia's President Vladimir Putin - perhaps under-estimating how seriously Britain took Litvinenko's murder, perhaps seeking to calm the situation - dismissed the whole spat as all rather silly.
But since the summer relations have not improved, in fact if anything they have deteriorated further - though trade ties continue to flourish.
Mr Lugovoi, whose extradition has foundered, has hit back with counter-accusations of "dirty tricks" by Britain's MI6, even suggesting it had a hand in Litvinenko's murder.
The Kremlin is deeply irritated by the presence in Britain of Mr Putin's opponents, like Boris Berezovsky (who appears to have survived an assassination plot this year).
Moscow has complained of a lack of co-operation in its own investigations in Britain.
In October, Britain annoyed Russia further by appointing the most famous KGB defector, Oleg Gordievsky, a Companion of St Michael and St George (CMG) in the Queen's Birthday Honours.
Russia responded a month later by awarding the MI6 defector George Blake the prestigious Order of Friendship.
Vladimir Putin has been dismissive of the spat
To anyone outside Whitehall this might all sound quite childish, a game of tit-for-tat where nobody wins.
But on 5 November Jonathan Evans, the director-general of the security service MI5, took the almost unprecedented step of publicly accusing Russia of spying on Britain and of taking up his organisation's time in countering this espionage when it needed to stop al-Qaeda.
"Since the end of the Cold War we have seen no decrease in the numbers of undeclared Russian intelligence officers in the UK - at the Russian embassy and associated organisations conducting covert activity in this country," he complained.
"So, despite the Cold War ending nearly two decades ago, my service is still expending resources to defend the UK against unreconstructed attempts by Russia, China and others, to spy on us".
Little wonder, then, that intelligence co-operation on counter-terrorism between London and Moscow is effectively frozen, and that mutual suspicions are back close to where they were in the dark days of the 1980s.