By Paul Reynolds
World affairs correspondent, BBC News website
Alexander Litvinenko died in hospital from radiation poisoning
A period of tense relations between Britain and Russia is expected following the British request for the extradition of a former KGB agent Andrei Lugovoi over the murder of Alexander Litvinenko.
However, the British government is determined that the criminal investigation should take priority over any diplomatic difficulties and is quite prepared for a delicate period ahead with Moscow as the extradition request takes its course.
Indeed, it will defend its position by sticking by what it regards as a classic case of the rule of law being applied in a criminal case. In this way, it hopes that Moscow will be forced onto the defensive.
"This remains a legal matter," a foreign office official told me.
The extradition request will be made under the 1957 Council of Europe European Convention on Extradition. It will be government to government. However, Russia has the right, under Article 6, to refuse to extradite one of its nationals. If that happens, the case is supposed to be referred to local authorities.
In any event, the Russian constitution does not allow the extradition of a Russian citizen. Article 61.1 says: "A citizen of the Russian Federation may not be deported from Russia or extradited to another State."
So the issue of extradition appears closed.
The argument will then move to whether Russia should hold its own trial. Under the European Convention, it should at least explore this. Under its constitution, it can do so.
Britain has to send a detailed account of the case so presumably that will include the evidence against Mr Lugovoi.
According to Martin Sixsmith, a former BBC correspondent in Moscow and author of a recent book on the case called The Litvinenko File, the British request for extradition will not succeed.
"Russia will say no. There is no extradition treaty so any extradition has to be agreed between the governments. Russia has made it clear that they will not comply with any request," he said.
"Russia claims that if the evidence is overwhelming, then it could try the case in its own courts, but this is very unlikely to happen. It has itself opened a case not only into the murder of Alexander Litvinenko but into the attempted murder of Dmitri Kovtun [another former Russian agent who also met Mr Litvinenko in London], thereby portraying Kovtun as a victim.
"However, their investigation is getting absolutely nowhere and they are using it as an excuse to come over and quiz the Russian exiles who live in the UK.
"They saw Boris Berezovsky [an exiled former oligarch who made millions during the Yeltsin era] who told me they had not even asked about Litvinenko but about his bank accounts, so that shows where their interest lies. Akhmed Zakayev [a Chechen exile accused of terrorism by the Russians] refused to see them. The whole thing is at an impasse, but at least the CPS has been brave enough to go through the motions.
"You have to see this whole thing as part of the war between President Putin and his supporters and their opponents, which has burst into the open.
"Specifically this was probably a revenge attack by the Russian FSB against Litvinenko, a former agent who blew the whistle on corruption in the FSB in 1998, though I think it probably only went up to colonel level for approval, not to the top.
"The polonium probably came from a government laboratory at Dubna not far from Moscow. The evidence in this case will be mainly circumstantial, based on the polonium trail, especially in places where Litvinenko himself did not go."
The Russians have not only opened their own case but have requested the extradition of Mr Berezovsky, who told the British newspaper The Guardian this year: "We need to use force to change this regime. It isn't possible to change this regime through democratic means. There can be no change without force, pressure."
The UK has refused to extradite Boris Berezovsky to Russia
The Russian argument is that Britain is harbouring people who advocate violence and that Moscow's request has sufficient merit to be accepted.
Britain has always refused to extradite Mr Berezovsky, who has been granted refugee status in the UK. However it has warned him not to involve himself in anything that could undermine his status and the foreign office is carrying out a second investigation into his activities. He was warned once before.
The whole series of incidents also has to be put against the backdrop of the currently strained relations between Russia and the West in general.
President Putin recently appeared to imply that US foreign policy could be compared to that of the Nazis.
There was a tense EU-Russia summit last week during which the German Chancellor Angela Merkel complained about protesters being prevented from even travelling to the venue.
Russia has complained about US plans to set up an anti-missile system in Poland and the Czech Republic, even though US officials say Russian defences would not be affected.
If Russia wanted to make life even more difficult for Western governments, it could block further sanctions on Iran over Tehran's nuclear activities, though it has always acted with caution. It could also hold out against a UN Security Council resolution on limited independence for Kosovo. But it has its own agenda in these areas, which might equally be unaffected by this dispute.
The main effect will probably be bilateral, with relations with Britain likely to go into cold storage for some time.