Britain warned Russia on Wednesday any attempt to intimidate staff of the British Council was "completely unacceptable" after Russia's state security service summoned local employees to speak to its officers.
Russia said the British Council's refusal to comply with an order to halt work from January 1 at its two regional offices was "dragging our relations into a yet deeper, systemic crisis".
Britain's consulate in St Petersburg said the British Council office in the northern city had been forced to shut temporarily because all its Russian staff had been told to report to the authorities for the second time in 24 hours.
Russia has demanded the British Council close two regional offices in a row that has its roots in a dispute over the 2006 murder of Alexander Litvinenko, a Kremlin critic and ex-Russian agent who died in London from a massive dose of radiation.
Britain has refused to close the Council offices - in St Petersburg and the Urals city of Yekaterinburg - and rejects Russia's allegation that they are operating illegally.
A Council spokeswoman in London said its staff in both cities had been summoned on Tuesday to see the Federal Security Service (FSB), the main successor to the Soviet-era KGB secret police.
The staff were also visited at their homes on Tuesday evening by Interior Ministry officials, she said. Russian officials said they were holding explanatory talks with Council staff, not questioning them.
"Any intimidation or harassment of officials is obviously completely unacceptable," Foreign Secretary David Miliband told reporters in London.
"The only losers from any attack on the British Council are Russian citizens who want to use the British Council ... and the reputation of the Russian government," he said.
