The "Vanguard" class were designed in the early 1980s by Vickers Shipbuilding and Engineering (VSEL) based at Barrow-in-Furness, now BAE Systems Maritime – Submarines. They were designed from the outset as nuclear-powered ballistic missile submarines, able to accommodate the UGM-133 Trident II missiles. As such, the missile compartment is based on the same system used on the American , which is also equipped with the UGM-133 Trident II. This requirement led to the "Vanguard"-class design being significantly larger than the previous Polaris-equipped "Resolution" class, and at nearly 16,000 tonnes they are the largest submarines ever built for the Royal Navy.