By Paul Reynolds
World Affairs correspondent, BBC News website
Stephen Kinnock, head of British Council in St Petersburg
In the diplomatic tit-for-tat over the Alexander Litvinenko affair, Britain is refusing to take one step open to it under a European convention governing extradition.
It could give the Russian authorities the evidence it has against the former KGB officer Andrei Lugovoi and ask them to take over the case themselves.
In theory, it could publish the evidence itself in any event. This would show to the world how strong its case is. Without such information, it is much easier for Russia to dismiss the extradition request as politically motivated.
However, the British government is afraid that the Russians might simply dismiss the evidence or use it to put Mr Lugovoi on trial, perhaps in secret, and then announce that he had been acquitted.
Either way, the proceedings would thereby be ended.
"He would not get a non-political trial in Russia," said a British official.
So the British policy is to continue with the extradition request. It probably knows it will not get anywhere, especially as Mr Lugovoi is now an MP - and is prepared for the consequences. These are unfolding, with the British Council now being involved and its St Petersburg director Stephen Kinnock being stopped by police.
"This is real Cold War stuff," said one Russian expert. "You have got to remember that there is huge suspicion of such foreign organisations in Russia and always has been."
Russia, in fact, has two legal instruments to refuse the request.
Under the 1957 Council of Europe Convention on Extradition, a state has the right, under Article 1a, not to extradite. There is a further obstacle, Article 61.1 of the Russian constitution, which says that a citizen of the Russian Federation "may not be deported from Russia or extradited... "
The British Council promotes cultural ties
The British ambassador to Moscow, Sir Tony Brenton, has suggested that a way round the constitution could be found. But it is difficult to see how a British government arguing the rule of law can suggest a circumvention.
Article 90.3 states that the Russian president cannot issue orders that "run counter" to the constitution. Only a change to the constitution could be effective and that is not realistic.
Council of Europe Convention
Article 6 of the European convention provides for a two-stage process. In the first stage, the extradition is requested. If the extradition is refused, the "requesting party" can then ask the "requested party" to refer the case to its own competent authorities, with a view to a trial being held by them.
The British government has only gone as far as stage one. It has not asked the Russians to take on the case themselves.
For the purposes of extradition, the Russians have been given only a summary of the case against Mr Lugovoi. Article 12 of the convention says that a "statement of the offences..."[their] time and place..[and] legal descriptions..." is all that is required.
For stage two, the actual evidence has to be handed over - the "files, information and exhibits relating to the offence..."
These are the crown jewels in the case. They would detail the evidence behind Britain's belief that Mr Lugovoi murdered Alexander Litvinenko by polonium poisoning - a charge Mr Lugovoi denies.
But if the impasse continues, they might never be known.