This was the designation of Allied convoys (together with a numerical suffix) plying the route from Belfast Lough or, in two cases, Holyhead, to Milford Haven (January 1941/May 1945).
The convoys of this series catered not only for local traffic but also for vessels arriving in Belfast from the North Atlantic convoys. The series is almost unique inasmuch as convoy numbers were reissued. The first of these 432 convoys was BB.1 of 22/24 January 1941 with five ships including the 6,997-ton British British Renown, and the last was BB.116 of 17/18 May 1945 with five ships. The convoy with the highest numerical indicator was BB.318 of 24/26 August 1943 with 10 merchant vessels.