- What is Trident?
Britain's Trident fleet, based in the river Clyde in Scotland, consists of four submarines that carry the nation's nuclear weapons system.
The Vanguard-class submarines can each carry eight nuclear missiles and its these missiles that give Trident its name.
- How do the four submarines work?
The Trident II D-5 ballistic nuclear missiles can each deliver a number of warheads although the Government hopes never to fire them and instead employs them as a deterrent to nuclear attacks.
The fleet works on rotation with one submarine always at sea somewhere in the world's oceans, while another one undergoes maintenance. The other two are used for training until they next rotate.
- What are the missiles' capabilities?
The missiles have a range of more than 7,000 miles. At their fastest, each missile can travel at over 13,000 miles per hour with an estimated power equivalent to eight Hiroshima nuclear bombs.
- Why is Trident in the news now?
The submarines, which are controlled from a Scottish base, are due to be replaced with Parliament set to vote on whether to renew or disarm Trident.
The £40 billion construction of a new fleet, Successor, could begin this year and be operational by 2028 while the current fleet will be phased out by 2032.
- Who is for or against renewal?
Political opposition to renewal comes from left-wing Labour MPs, most notably leader Jeremy Corbyn, the SNP, the Greens and Plaid Cymru, who view it as an outdated and unnecessarily expensive relic of the Cold War era.
A majority of Labour and Conservative MPs are expected to vote to renew Trident, though, viewing it as a still-necessary strategic expense in Britain's defences.
The Prime Minister consents to the firing of the nuclear missiles in a "letter of last resort" carried on the four submarines in the event of Britain being annihilated by a nuclear strike as part of a strategy of mutually assured destruction.