What is Britain's nuclear defence capability?

In October 1952, the United Kingdom tested its first-ever nuclear explosive device, becoming the third state to develop a nuclear weapon capability – behind the United States and the Soviet Union.

Now with an around-the-clock, at-sea deterrent, the UK is now recognised as one of five nuclear states under a United Nations pact.

Alongside the four other permanent council members – France, Russia, the US and China – the UK operates under the Nuclear Non-Proliferation Treaty (NPT).

An active disarmament deal since 1970, the NPT had seen the UK commit to a 65% reduction in nuclear stockpiles from the end of the Cold War to the mid-2020s.

As part of the Integrated Review of Security, Defence, Development and Foreign Policy 2021, the UK will now move to an overall increase of the nuclear weapon stockpile to "no more than 260 warheads" – citing the development of emerging threats.

At its Cold War peak, UK defence possessed approximately 520 nuclear warheads.

Currently, up to four British nuclear submarines, based at Scotland's HMNB Clyde, patrol global waters, carrying Trident thermonuclear warheads as part of the UK's continuous at-sea deterrent.

The Vanguard-class vessels are due to be replaced by the incoming Dreadnought-class in the 2030s – set to be the largest submarines ever operated by the Royal Navy.

Four of the vessels are set to cost the UK more than £30bn to produce.

Following NATO policy, the UK uses minimal credible nuclear deterrence, but does not follow a policy of 'no-first use' – which would prevent launching a nuclear attack unless first coming under nuclear attack from another state.

It has vowed not to use or threaten a nuclear attack against any non-nuclear state within the NPT unless the agreement is breached.

Of the five nuclear weapon states, the UK is alone in having only one single deterrent system and has announced it will replace its Mk4 nuclear warhead.

In conjunction with the US, a Common Missile Compartment (CMC) for the Dreadnought programme is being built, while the UK also has a £10bn back-up plan in case the submarine modernisation falls through.

Cover image: HMS Vanguard's DASO Trident missile being launched in 2005 (Picture: MOD).