The Federal Security Service (FSB) said retired officer Sergei Skripal, 55, who was convicted of "treason in the form of espionage", had been recruited by the Secret Intelligence Service, better known as MI6.
"The investigation into the case ... established that during his service in the Russian Armed Forces in the mid-1990s, Skripal was recruited by British intelligence and shared state secrets with it," the FSB said in a news release.
The FSB also said that after he retired from military service in late 1999, Skripal continued to cooperate with MI6 and met with its representatives overseas.
"Skripal had received the secret information that he reported to the British services from former colleagues after leaving the military," the FSB said. "His actions caused serious damage to the national defense and security."
The FSB also said that MI6 officials had paid Skripal for the information in foreign currency, which was transferred monthly to his account in a Spanish bank.
"During the investigation, Skripal admitted his guilt and gave truthful testimony about his criminal activities, which the court had taken into consideration," the FSB said.
Chief Military Prosecutor Sergei Fridinsky said he was satisfied with the verdict, adding that Skripal would serve his sentence in a high-security penal colony.
"Skripal was accused of spying for foreign intelligence services," the prosecutor said, adding that Russia's security services would definitely be able to repair the damage Skripal did to the state.
Fridinsky said he had asked the court to sentence Skripal to 15 years, but the court had taken into account mitigating circumstances such as the convict's admission of guilt, assistance to the investigation and poor health.