By Paul Reynolds
World affairs correspondent, BBC News website
The British have asked for the extradition of Andrei Lugovoi
Accusations by senior British security sources that the Russian state backed the murder of Alexander Litvinenko shows that this issue continues to bedevil Russian-British relations.
The sources told the BBC's Newsnight programme that "there are strong indications that it was state action... and not a rogue element".
The use of lethal polonium "is evidence of state involvement", the sources said. After all, it is not a substance you can pick up at a department store.
The accusation is not fundamentally new. Nobody involved in the request to Russia for the extradition of Andrei Lugovoi has any doubts that only a state organisation could have ordered such an operation and provided the special radioactive means to carry it out.
The only issue was how far it had Kremlin approval.
The conclusion to be drawn from these latest comments is that Britain believes that the Russian security service the FSB was operating largely independently, but in a permissive environment in which the Kremlin signalled that action against critics was allowed.
The Russian parliament even passed a law making it legal to carry out assassinations abroad.
But going public with an accusation like this (and another one that the Russians scouted out another London-based critic, Boris Berezovsky) is another matter and it will raise the diplomatic temperature.
Whether the comments were timed deliberately or whether they just came out in the course of a conversation with the BBC journalist is not clear. Sometimes, these revelations are not the result of a careful decision but just emerge and have all kinds of unexpected repercussions.
It is a reminder that all is certainly not well in the relationship between Russia and Britain and, beyond that, between Russia and the West in general.
It will dash any hopes that the meeting at the G8 in Japan between the two leaders, Gordon Brown and Dmitry Medvedev, might have started a new chapter.
The talks were in any case, according to a Russian official, "sharp" at times as Mr Brown raised the various issues clouding the relationship - the outstanding extradition request, the restrictions on the British Council and the Russian manoeuvres against BP.
It is not just the Litvinenko affair that is spoiling the picture.
But it will continue as a problem because Britain will not withdraw its extradition request and Russia will not agree to it.
Russia in fact has two legal locks against sending Mr Lugovoi to the UK. The first is that the 1957 European extradition convention, under which the British request was made, gives a state an absolute right to refuse a request (Article 1a). It does not have to give a reason.
The second is that the Russian constitution says that a Russian citizen "may not be deported from Russia or extradited..." (Article 61.1).
Britain could further escalate the dispute by revealing the evidence it has against Mr Lugovoi. This would help back up its accusations by providing the world with the background, including the radioactive trail through London.
It could alternatively, under the convention, ask the Russians to take on the prosecution themselves. However the British fear is that this would lead to an acquittal, in which case Mr Lugovoi might be free to travel without fear of arrest, as he cannot at the moment.
There is therefore an impasse and no way out yet.