British Security Service (M15) attempt in 1955 to plant a microphone in the Soviet Consulate in the Bayswater area of London. When the building next door was being renovated, M15 operatives, under the cover of decorators, drilled through the 18 inches of wall between the two buildings to push a pinhole into a consulate conference room and install a highly sensitive microphone.

The bug was successful for about six months and then suddenly ceased to operate. The pinhole opening had apparently been detected and filled in. A British agent was later able to determine that a soundproof second wall had also been installed over the wall where the bug had been planted and that the room had not been used for conferences.

Most likely, one of the Cambridge Spy Ring had learned about the microphone and tipped off the Soviets, who promptly took countermeasures.