In February 1941 the Ministry of War Transport agreed to reserve ships then under construction for possible conversion to escort carriers, although they refused to release existing ships. The one exception was Activity, who was completed in August 1942 as the refrigerated cargo carrier Telemachus (ex-Empire Activity), and operated by the Alfred Holt Line.
Campania, Nairana and Vindex were all converted from refrigerated cargo ships, while Pretoria Castle was converted from a passenger liner.
These were "naval" escort carriers as opposed to the "merchant" escort carriers of the "Empire" and "Rapana" Classes.
Unlike the American conversions, which had short wooden flight decks, and many of which had open-sided hangars and a single screw, these British conversions were all twin-screw vessels with enclosed hangars and relatively long steel flight decks. However, they still had only one lift.
The five were to have had identical conversions, but Campania, who was not launched until 17 June 1943, received several improvements not fitted in the other four ships. She received the first Action Information Organization (AIO) suite to be fitted in a British carrier, and also had Type 277 radar which gave height indication and good coverage at low level. Her modern air direction room (ADR) and radar outfit proved invaluable for directing both her own and other carriers' aircraft.