A retired Russian intelligence officer was sentenced to 13 years in jail today for passing state secrets to Britain's MI6 and betraying dozens of Russian spies working in Europe in the late 1990s.

Prosecutors said that Colonel Sergei Skripal, 55, worked for the UK foreign intelligence service for several years after being recruited while on a posting abroad.

Investigators said that he revealed the name of several dozen Russian agents working around Europe to MI6, which paid him a total of around $100,000 into a Spanish bank account before he was arrested in December 2004. The agents he named were all sent back to Russia.

Earlier this year, British diplomats in the Russian capital were caught up in an embarrassing Cold War-style spy scandal when the Federal Security Service (FSB) - the main successor to the Communist-era KGB - accused four diplomats of using a hollowed-out rock left in a suburban park to communicate with local agents.

In intelligence terms, however, the Skripal case would appear to be much more significant. Russian agency and newspaper reports said that Skripal had been working for the Armed Forces when he was "turned" by MI6 and one report described him as a "military intelligence officer".

That would mean that Skripal had been an officer not of the FSB or KGB but of the GRU, Russia's military intelligence agency.

Oleg Gordievsky, one of the highest-ranking KGB defectors of the Cold War who was spirited out of Russia by British intelligence in 1985 and now lives in Britain, said that if those reports were true, Skripal may have been an agent of "exceptional importance".

"The countries they operate in know already who is KGB, who is GRU. Betraying the names is the least interesting thing," Mr Gordievksky, the former KGB station chief in London, told Times Online.

"What the British and Americans want to know is the military strategy things and if he was a GRU officer then he was in the military. In the current situation, it is more important to be GRU than KGB, because the KGB doesn't know about military matters, troop deployments, nuclear co-operation with Iran and so on... That's why, if it's true that they had a GRU agent for several years, then we should congratulate them."

Russian officials did not spell out which branch of Russian intelligence Skripal worked for.

Yevgeny Komissarov, a spokesman for the Moscow military court where he was tried but which only made the case public today, said: "According to the court, Sergei Skripal is guilty of state treason through the transfer of state secrets ... to the British secret service."

Sergei Fridinsky, Russia's chief military prosecutor, said in comments broadcast on state television after the verdict that it was "impossible to measure in roubles or anything else" the amount of harm that Skripal caused to the Russian state. "The conviction of Skripal is lawful and justified," he added.

The FSB said in a news release quoted by the RIA-Novosti news agency that Skripal was recruited by MI6 in the mid-1990s while serving in the Russian armed forces. After his retirement in late 1999, he continued to co-operate with MI6 and met its officers in Moscow.

"Skirapl 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 national defence and security."

The MI6 agent had faced a maximum of 15 years but received a slight reduction in his sentence because he admitted his guilt and cooperated with investigators. He will serve his sentence in a high-security penal colony.

The Skripal case is not thought to be connected with the "spy rock" row in Moscow earlier this year, which was dismissed by some Kremlin critics as part of a campaign to discredit Russian non-governmental organisations working for human rights and democracy.

The FSB, the main successor agency to the Communist-era KGB, said at the time that a Russian citizen who allegedly had contacts with British agents had been detained and confessed to espionage.

Since the election in 2000 of President Putin, himself a former KGB colonel and one-time head of the FSB, prosecutions for espionage have markedly increased in Russia. In 2004, an arms control researcher, Igor Sutyagin, was convicted of treason for allegedly selling information on nuclear submarines and missile-warning systems to a British company that investigators claimed was a CIA cover.

A physicist, Valentin Danilov, was also convicted that year of selling classified information on space technology to China.

Mr Gordievsky, who was first recruited by MI6 in 1968, said of the case: "The big question, if a GRU officer is arrested, and if he had been a British agent, is 'who betrayed him?'"

Referring to the post-war spy who betrayed dozens of British agents in the Eastern Bloc, he added: "That's a very important question, because God forbid we have another George Blake."

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