Britain has denied claims that its agents were behind the murder last November of Alexander Litvinenko. The accusation came from another ex-KGB agent, Andrei Lugovoi, sought by Britain in connection with the murder.
Andrei Lugovoi is seeking to turn the tables on his British accusers
BBC Security Correspondent Gordon Corera looks at the row.
Andrei Lugovoi's claims are being viewed by officials in London as an attempt to further muddy the already murky waters surrounding the Litvinenko case.
Anything to do with spies, after all, is bound to attract headlines.
It is not impossible that Britain's foreign intelligence service, MI6, did talk to Alexander Litvinenko and Andrei Lugovoi for the purposes of gathering intelligence, as the latter has claimed.
That is, after all, the job of an intelligence service.
Russian and British intelligence agencies have also recently been trading blows reminiscent of the old Cold War days at a time when Russian-Western relations generally have been deteriorating.
Alexander Litvinenko died in hospital from radiation poisoning
The Russians exposed British agents and their use of a fake rock to transmit information, while UK officials have said Russian activity in the Britain is back to the levels of the Cold War.
But as a matter of principle, MI6 never confirms or denies the identity of those who work for it and the further assertion that all of this means MI6 was somehow involved in Alexander Litvinenko's murder stretches credibility - it is argued in London - particularly on the issue of motive.
Killer or scapegoat?
It has also been noted in London that Mr Lugovoi's accusations closely echo the line coming from the Russian government since the start of the investigation - particularly the allegations that exiled Russian tycoon Boris Berezovsky could have been involved.
Mr Berezovsky denied being an MI6 agent.
"The UK authorities know very well who their MI6 agents are in the UK and so they know that I am not one of them," he said in a statement.
"Following Andrei Lugovoi's press conference in Moscow this morning, it is now clearer than ever that the Kremlin is behind the murder of Alexander Litvinenko. Everything about Mr Lugovoi's words and presentation made it obvious that he is acting on Kremlin instruction."
Sources close to the police investigation have always maintained that the evidence pointing to Mr Lugovoi - based largely on the trail of radioactive polonium and its coincidence with his presence - was strong and led to the decision by the authorities to publicise a desire to press charges.
Mr Lugovoi described himself as a "victim not a perpetrator of a radiation attack" while in London, and said the charges against him were politically motivated.
He told the Moscow news conference that MI6 had recruited Mr Litvinenko and had also tried to recruit him to collect information on Russian President Vladimir Putin.
British officials are emphasising that this is a legal matter relating to a criminal offence - not an intelligence issue - and that they await a formal response from Russia to the request for Mr Lugovoi's extradition.
He did not directly answer questions in the news conference about whether or not he would be willing to come to London.
And Thursday's claims and counter-claims seem to indicate that a trial remains a distant prospect, while a diplomatic - and perhaps even an intelligence row - is a much closer one.