On 9 September 1940, "Aaron Ward" was decommissioned at Halifax, Nova Scotia. Transferred to Britain as one of the 50 old destroyers leased to that nation in return for the right to establish American bases on British possessions in the Western Hemisphere, she was commissioned in the Royal Navy that same day as HMS "Castleton". "Castleton" was modified for trade convoy escort service by removal of three of the original /50 caliber guns and one of the triple torpedo tube mounts to reduce topside weight for additional depth charge stowage and installation of a Hedgehog anti-submarine mortar system.