Russia said today it would expel four British diplomats, stop issuing visas to British officials and cease co-operation with London in the "war on terror" after Britain's decision to send home four Russian envoys.
The row was prompted by Russia's refusal to extradite a murder suspect to face trial in London.
A Foreign Ministry chief spokesman said the British ambassador had been summoned and handed a note "in connection with the unfriendly actions of Britain towards Russia".
"Four British embassy staff in Moscow are now persona non grata and they should leave the territory of the Russian Federation within 10 days," he added.
"The position towards Russia taken by the new Labour government of Great Britain is based on anything but common sense or pragmatism and respect of law, which is so typical of real Britons," the spokesman added.
British prosecutors want Russia to extradite former KGB agent Andrei Lugovoy to face trial for poisoning anti-Kremlin émigré Alexander Litvinenko's tea with deadly radioactive polonium during a meeting in a London hotel last year.
Moscow says its constitution bans extradition, and President Vladimir Putin has called the request "stupid".
Russian officials have responded indignantly to British suggestions that they should change their constitution to accommodate the request, pointing out that Britain has turned down numerous Russian extradition requests in recent years.
Mr Lugovoy has appeared in front of the media in Russia insisting he is innocent and accusing British intelligence of having a hand in the murder of Mr Litvinenko.