What is trident?

Trident is the UK's nuclear weapons system. It uses Trident II D5 missiles carried on four Vanguard-class submarines based at Faslane in Scotland. At least one submarine is always on patrol, a policy called continuous at-sea deterrence. Trident is meant to deter attacks by guaranteeing the ability to retaliate.

The UK has kept a nuclear submarine on patrol at all times since 1969. Trident replaced the earlier Polaris system in the 1990s. The missiles are built in the United States and shared with the US Navy, while the warheads are designed and made in the UK. The Prime Minister is the only person who can authorise their use. It costs billions each year.

The four Vanguard submarines are due to be replaced by four new Dreadnought-class boats from the early 2030s. The government has set aside £31 billion for the programme, plus a £10 billion contingency. Work is also under way on a new warhead. In 2025 the UK announced it would buy F-35A jets able to carry US nuclear bombs as part of NATO's mission.

Supporters argue Trident is the ultimate insurance policy for national security and a key UK contribution to NATO. Opponents argue the weapons are immoral, very expensive, and that the money would be better spent on conventional forces or public services. The SNP opposes having the weapons in Scotland. The defence nuclear enterprise takes about a fifth of UK defence spending. Both Labour and the Conservatives back renewing it.