Domesday Live

Why does the UK have nuclear weapons?

The UK keeps a small nuclear force on submarines as a deterrent: the idea is that an enemy would not attack, knowing it would face a devastating reply. The current Trident system is being replaced with new Dreadnought-class submarines, a programme costing tens of billions of pounds. Supporters see it as the ultimate insurance, while opponents say it is costly and makes no one safer.

Why it happens

• A deterrent, not for use. The aim is to prevent an attack. A submarine always on patrol, hidden at sea, means no enemy could hope to destroy the UK's ability to reply.
• How it works. Trident missiles, supplied under an agreement with the United States, are carried on British-built submarines based at Faslane in Scotland, with at least one on patrol at all times.
• Replacement is under way. The ageing Vanguard-class submarines are being replaced by new Dreadnought-class boats, with the first due in the early 2030s.
• The cost. The new submarines and the warheads that go with them cost tens of billions of pounds over their lives, and the UK is one of nine countries with nuclear weapons.
• A promise to work towards disarmament. As a member of the Non-Proliferation Treaty the UK has agreed to work towards a world without nuclear weapons in the long term, which opponents say its plans do not match.