The announcement came a day after state television channel Rossiya broadcast footage purportedly showing four British Embassy staff using electronic equipment concealed in the rock in Moscow to receive intelligence from Russian agents.
A prominent rights activist warned the accusations could be used as a pretext to crack down on Western-funded groups that are critical of the Kremlin.
Sergei Ignatchenko, a spokesman for the Federal Security Service, the main successor to the Soviet-era KGB, said the situation would be resolved "at a political level," the RIA-Novosti news agency reported, an apparent indication that the Russian government could expel the diplomats.
The intelligence agency also said a Russian citizen who allegedly had contacts with British agents had been detained and confessed to espionage, according to the Russian Interfax news agency.
Officials at the British Embassy in Moscow and Foreign Office in London declined to comment on the espionage accusations. Prime Minister Tony Blair said at a news conference that he had only heard about the allegations in media reports and had no further comment.
In addition to a post-Cold War chill in Russian-British relations, the announcement reflected a toughening Russian attitude toward NGOs. Earlier this year, President Vladimir Putin signed a law severely restricting NGOs' financing and activities.
Moscow has been highly suspicious of groups promoting human rights and democracy since opposition leaders came to power in recent uprisings in the former Soviet republics of Georgia, Ukraine and Kyrgyzstan. Russian officials have accused Western nations of encouraging regime change in the regions by financing NGOs.
Martin McCauley from the Institute of Slavonic Studies at University College London told CBS News correspondent Mark Phillips Putin is worried about the information being conveyed.
"One thing they're concerned about is information which would reveal the country is not entirely stable," McCauley said. "That the opposition is stronger than President Putin would like to admit."
Rossiya said the diplomats had downloaded information onto handheld computers from the electronic gadget hidden in the rock, a process that worked at a distance of up to 65 feet and took only one or two seconds.