The Russian security services have arrested two people it claims are spies for British Intelligence.
It is not clear if their alleged activities are linked to recent Russian claims that British spies were using a computer hidden inside false rock to collect and relay information from Russian informants.
In London, a British Foreign Office spokesperson said it was asking the Russian foreign ministry for information.
The affair coincides with moves by President Vladimir Putin to clamp down on NGOs which Kremlin officials accuse of promoting Western political interests in Russia.
Putin, himself a former KGB spy, said on Wednesday he had not decided whether to expel the diplomats who were named.
