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.
After meeting the head of the British diplomatic service, Peter Ricketts, the Russian ambassador signalled Moscow would not soften its line, saying the situation was hurting relations.
"Namely the British side, which initiated the worsening of our relations last summer, must now think seriously about the consequences that may follow if it does not fulfil the Russian authorities' decision to close the British Council offices," Yuri Fedotov said in remarks shown on Russian television.
TIES AT LOW EBB
Relations between Britain and Russia are at their worst since the Cold War, soured by the Litvinenko murder, mutual espionage allegations and political asylum given by Britain to prominent enemies of President Vladimir Putin.
On December 12 Russia ordered the British Council to halt work from January 1 at its two regional offices, saying Britain had broken a host of international and domestic rules.
Russia's Interfax news agency quoted a source in the FSB as saying the Council's staff were not subjected to questioning.
"The ... employees talked to federal security officers, who explained to them that the British organisation is operating illegally," it said.
The FSB said late on Tuesday it would speak to the staff to prevent them from being used by Britain "in provocative games".
A Reuters photographer outside the British Council office in St Petersburg said the premises were deserted. A recorded telephone message said its closure was "due to the recent action against us by the Russian authorities".
The Council spokeswoman said the organisation was "deeply concerned" about the treatment of its staff. "Our main concern is the safety and security of our Russian and UK staff."
She said Stephen Kinnock, head of its St Petersburg operations and son of former European Commissioner Neil Kinnock, had been stopped by police on Tuesday evening for a traffic violation. He was given a ticket and released.
