The lengthy operation reportedly failed to yield any proof, and the suspected double agent eventually quit the agency

British intelligence agencies spent two decades hunting a suspected Russian double agent inside MI6, only to drop the case after failing to find evidence, The Guardian has reported, citing unnamed sources. The investigation, dubbed Operation Wedlock, was eventually closed as "inconclusive," and the suspect left service, the outlet said on Friday.

The probe, which reportedly ran from the mid-1990s to around 2015, was led by MI5, the UK's domestic counter-intelligence agency. MI6, which is responsible for foreign intelligence, launched the inquiry after a CIA tip-off suggested a senior officer may have been spying for Russia.

"[We were told] the target was a Russian spy... The US believed he was leaking information to the Russians," a source told The Guardian. "The job was taken more seriously than any other [MI5] was involved in. Wedlock eclipsed them all."

The operation allegedly involved some 35 officers. MI5 is said to have bugged the suspect's home, tailed him around London with video surveillance, and even followed him abroad, despite that being outside its legal mandate. MI5 reportedly believed the mole had two accomplices based in London but found no supporting evidence.

"We thought we had another Philby on our hands," a source told the outlet, referring to Kim Philby, one of the most prominent members of the intelligence group Cambridge Five, which supplied the Soviet Union with information from the UK during World War II and the early stages of the Cold War.

The Wedlock spy saga is believed to have dragged on until at least 2015, by which time the suspect had left MI6. Despite the scope of the operation, MI5 failed to prove any espionage.

"MI5 never got the conclusive proof it was looking for," one source said, calling Wedlock "highly unusual... the longest in recent memory and probably the most expensive."

The UK has repeatedly accused Russia of espionage and sabotage in Europe without offering proof. In a high-profile case in 2018, London alleged Moscow tried to poison ex-Russian military intelligence officer turned MI6 asset Sergey Skripal and his daughter. Russia denied involvement.

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Tensions between Moscow and London have worsened since the escalation of the Ukraine conflict in 2022. The UK is one of Kiev's staunchest backers, imposing sanctions on Russia and supplying weapons to Ukraine. Russian officials believe that British intelligence trains Ukrainian units for sabotage missions inside Russia.