MI5 has been called in to examine whether a spy suspected of being one of Russia’s top agents sold highly classified British secrets to Moscow.

Officers from MI5’s counter-espionage section are investigating possible damage caused to Britain and the West by Herman Simm, a high-ranking Estonian security official who has confessed to spying for Russia.

In an operation worthy of a 1960s spy novel, Simm is reported to have used an antiquated radio transmitter to send classified information to Russia.

He is accused of betraying Nato plans for Kosovo’s independence; the position of the alliance on last summer’s war between Russia and Georgia; and secrets from the American missile shield in Europe.

A senior Whitehall official said MI5 “had visibility” of the Simm case and was in close touch with Nato investigators.

MI5 officers are trying to establish what contact Simm had with British officials and what access he had to military and industrial secrets supplied by the UK.

“MI5 will want to establish along with others in Nato potentially what might have been compromised. We have to work out where the person’s been and what [he’s] had access to. It will take a while to unpick what damage, if any, has been caused,” the official said.

Simm served as Estonia’s chief of police until 1994 before moving to the ministry of defence. He was head of the state secret protection office, tasked with protecting the country’s most sensitive secrets. In 2003 the Estonian prime minister issued him with a diplomatic passport, which allowed him to carry classified documents overseas.

In his latest post he was head of security in the defence ministry, responsible for liaison with Nato and the EU. He set up and ran the system for handling all classified information, including all secret documents shared between Nato countries. He oversaw security clearance for Estonian officials in the intelligence, military and security agencies.

Simm signed bilateral agreements with other EU countries for the exchange of classified information on military, commercial and industrial secrets.

Colleagues said he kept a low profile. One described him as “quiet and introverted to the point of possible eccentricity”.

Simm and his wife Heete, a lawyer in the police department, were arrested on treason charges in September. He is alleged to have received millions of dollars from the SVR, Russia’s foreign intelligence service, and, under the Soviet Union, from the KGB.

Simm is due to go on trial next year and could face a jail term of up to 15 years.

Additional reporting: Nicola Smith

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