Exclusive: Operation Wedlock lasted up to 20 years and took MI5 teams across world amid panic about ‘another Philby’

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.

The extensive hunt for the alleged mole, called Operation Wedlock, was run by MI6’s sister agency, MI5, which deployed a team of 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, it’s understood, because the officers travelled to the country without the knowledge of its government, and would have been illegal under international law.