The Successor programme refers to the replacement of the existing UK Trident programme of submarine-launched nuclear missiles. This would involve replacing the Vanguard class of four Trident ballistic-missile armed submarines with a new class designed to continue a nuclear deterrent after the current boats reach the end of their service lives. The government has begun planning a new submarine-based system but there is opposition from those who want to take the opportunity for full nuclear disarmament or replacement with a cheaper, less capable nuclear weapon delivery system. On 18 May 2011 the British government approved the initial assessment phase for the construction of new Trident submarines.