The FSB said it had confiscated a fake rock containing electronic equipment used for espionage on January 23, and had uncovered a ring of four British spies who worked under diplomatic cover, funding human rights organizations operating in Russia.
Russian officials have cited the espionage scandal to back their case for tougher financial reporting requirements for NGOs.
The State Duma, the Russian lower house of parliament, passed a law tightening NGO regulations late last year. The move stirred up criticism in the West, which said it would hamper foreign NGOs' activities in Russia. Russian officials in turn argued that Western countries' controls over foreign NGOs on their own territory was much more stringent.
The electronic "rock" allowed agents to upload classified computer data, which could then be downloaded by British embassy employees.
The FSB also said it had identified a Russian national contracted by the British intelligence services while on a business trip abroad where he had been supplied with spying devices. He was arrested and gave testimony, it said.
The park where the fake rock was planted was frequently visited by embassy officials last autumn, according to the FSB.
"There were obviously some problems with the electronic equipment hidden inside the rock. This explains the British agents' frequent visits to the area," the FSB suggested.
Marc Doe, a British Embassy official implicated in the scandal by the FSB, oversaw the activities of the UK Foreign Office's Global Opportunities Fund, authorizing regular payments to NGOs.
Several documents signed by him were shown as evidence of cash payments to NGOs, including 23,000 pounds (about $40,000) to the Moscow Helsinki Group, and 5,719 pounds ($9,700) to the Eurasia Foundation.