Continual tests took place over the next four years using both the Clausen Rolling Platform at RAE Aberporth and the "Girdle Ness". A final series of tests at sea, which culminated in sixteen successful firings, finally cleared the missile for service in 1961. After more than 250 launches, the Seaslug Mark 1, also known as Guided Weapon System 1, or GWS.1, finally entered service in 1962 on County-class, each fitted with a single twin missile launcher and a complete weapon system with one fire control set and 30 missiles. The Seaslug-armed cruisers were cancelled in 1957.