In 1924 the British Secret Intelligence Service adopted Passport Control Offices as a convenient cover for its stations overseas. Invariably the true role of the passport control officer was declared to the local security apparatus, and he or she acted in a liaison role with his hosts. By limiting their activities to the collection of information against a common adversary and taking care not to compromise British diplomatic interests, the PCOs generally operated against neighboring countries and thereby avoided undermining the relationship with the local regime or attracting unwelcome attention.
The PCO system was widely acknowledged to be semitransparent, which had the advantage of enabling potential informants to approach an SIS officer directly without embarrassing regular diplomatic personnel, but also meant that adversaries had an easy target to work on when planning penetrations or double agent operations. In the aftermath of the Venlo incident in November 1939, it became clear that the SIS station in The Hague had been hopelessly compromised and penetrated for years. Nevertheless the system was retained until the end of World War II, when the Foreign Office accepted the need to provide posts in overseas diplomatic missions for SIS personnel, thereby granting them a measure of protection under the terms of the Vienna Convention.