The "Y" service was a network of British signals intelligence collection sites, the Y-stations. The service was established during the First World War and used again during the Second World War.[1] The sites were operated by a range of agencies including the Army, Navy and RAF, and the Foreign Office (MI6 and MI5). The General Post Office and the Marconi Company provided some receiving stations, ashore and afloat. There were more than 600 receiving sets in use at Y-stations during the Second World War.[2]