LONDON

Britain's intelligence services secretly launched one of the most sensitive internal investigations since the Cold War amid fears that a senior MI6 officer had been spying for Russia, it has emerged.

Codenamed Operation Wedlock, the covert mission was initiated in the mid-to-late 1990s after a tip-off from the American Central Intelligence Agency (CIA) warned that a British intelligence official based in London was allegedly leaking classified material to the Kremlin.

The unprecedented probe was led by MI5, the UK's domestic security agency, and is understood to have lasted for nearly two decades—well into the 2010s.

At the center of the investigation was an unnamed MI6 officer, identified by US intelligence as "suspect 1A," who was believed to be a double agent.

According to the Guardian, which first reported on the issue, the operation mobilized up to 35 MI5 officers, including surveillance experts, analysts, and planners.

The intelligence teams conducted global operations, including a dangerous mission to the Middle East, where MI5 officers operated without the knowledge or consent of the host nation—a move that would have breached international law had they been discovered.

During the early phase of the operation, Russia’s FSB intelligence agency was headed by none other than Vladimir Putin, raising the stakes and heightening fears over the potential depth of the breach.

The extraordinary level of secrecy meant some MI5 officers were briefed on the operation in a church, away from government offices.

Others were initially misled into thinking they were taking part in a training exercise, only to be told the real nature of the mission once they had left MI5 headquarters at Thames House in Westminster.

To protect the operation’s integrity, the surveillance team operated out of a nondescript building in Wandsworth, South London, under the cover of a fictitious private security company. The site was chosen for its proximity to MI6’s headquarters at Vauxhall Cross.

Despite the scale and duration of Operation Wedlock, MI5 was ultimately unable to prove whether the suspected officer had passed secrets to Moscow. By 2015, the officer in question had already left MI6.