A spy at the heart of NATO may have passed secrets on the U.S. missile shield and cyber-defense to Russian Intelligence, according to Estonian officials.

Herman Simm, 61, an Estonian defense ministry official who was arrested in September, was responsible for handling all of his country's classified information at NATO, giving him access to every top-secret graded document from other alliance countries.

He was recruited by the Russians in the late 1980s and has been charged in Estonia with supplying information to a foreign power.

Several investigation teams from both the EU and NATO, under the supervision of a U.S. officer, have flown to the Estonian capital Tallinn to assess the scope of what is being seen as the most serious case of espionage against NATO since the end of the Cold War.

“The longer they work on the case, the more obvious it becomes how big the impact of the suspected treachery really is,” according to Der Spiegel magazine. A German official described the Russian penetration of NATO as a "catastrophe".

"Simm became a proper agent for the Russian government in the mid-1990s," said Jaanus Rahumaegi, who heads Estonia's parliamentary control commission for security services.

On the face of it, the Simm case resembles the old-fashioned Cold War spy story. He used a converted radio transmitter to set up meetings with his contact, apparently someone posing as a Spanish businessman.

As in the 1950s and 1960s, it seems that the operation was a husband-and-wife team. His wife Heete – who previously worked as a lawyer at the national police headquarters – has also been detained on charges of being an accessory to treason.

Click here for more on this story from the Sunday Times of London.