The class was designed as a hybrid cruiser-destroyer, with a much larger displacement -similar to that of the "Dido"-class cruiser - than its predecessor, the . In 1955 the new First Sea Lord Louis Mountbatten specified the development of an April 1955, 4800 ton Fast fleet escort design (DNC 7/959) as a Seaslug carrying vessel with a Y stern positioned twin 3/70 AA mount replaced by Seaslug (DNC 7/1002) instead of the then approved development of a large GW 58A 15,400 ton cruiser which would have combined Seaslug with Type 984 3D radar and a conventional "Tiger"-class gun armament. During 1956–1958 a full "alternative" gun armament was an option for the new GW Fast Escort, based on a modern combined gas turbine and steam turbine (COSAG) propulsion unit, as enlarged "Daring" fleet escorts, armed with two twin Mk 6 4.5-inch guns, two twin L/70 40mm Bofors and a twin 3-inch/70 guns. A detailed March 1957 study opted for a medium tensile long hull and a fit of 18 Seaslug and 4 special (nuclear) Seaslug for extended range AA, anti-missile and anti-ship. twin Mk 5 Bofors 40mm were maintained with the future and effectiveness of the "Green Light" (which would become Seacat) missile, under doubt and Limbo mortar was the only anti-submarine weapon.