Panic over 'another Philby' led to huge operation to find suspected Russian double agent in MI6

Britain's spy chiefs were forced to launch one of the most sensitive and risky investigations since the cold war over fears a senior officer at the foreign intelligence service MI6 was a double agent for Russia.

- Nick Hopkins Anna Isaac

The extensive hunt for the alleged mole, called Operation Wedlock, was run by MI6's sister agency, MI5, according to a source with close knowledge of the operation. It deployed up to 35 surveillance, planning and desk officers, who travelled across the world.

One trip took an entire surveillance team to the Middle East for more than a week, the Guardian has been told, where the officers were put up in a CIA safe house. This trip was particularly hazardous because the officers travelled to the country without the knowledge of its government, and would have been illegal under international law.

The investigation, which began in the 90s, is believed to have lasted in one form or another for up to 20 years, but MI5 could not establish whether British intelligence had a mole.

"We thought we had another Philby on our hands," said a source, referring to Kim Philby, the infamous MI6 double agent who was part of a group of Britons, recruited by the Soviet Union, known as the Cambridge spy ring.

MI6, the Secret Intelligence Service, is the UK spy agency responsible for overseas intelligence collection and agent handling; MI5, the Security Service, is the domestic intelligence agency that assesses threats to Britain's national security.