Gordon Brown’s declaration on Wednesday night that he backs renewal of Britain’s nuclear deterrent will be welcome news for BAE Systems, Britain’s biggest arms manufacturer and builder of the current fleet of four Trident submarines. But questions remain as to how the long-term commitment will be paid for.

Estimates suggest a full replacement of the four Vanguard-class submarines, with Trident missiles and warheads, could cost £15bn-£20bn. The current fleet cost about £13bn to build. By the time it reaches the end of its life, expected to be by 2024 at the latest, it will have cost up to £24bn, equivalent to 3-4 per cent of the defence budget in each of its years in service.