The main political parties are engaged in a bitter row around the replacement of the Trident nuclear deterrent - but what is it, how much will it cost and what are the options?
Key facts about Trident:
It is complete system of submarine-based nuclear missiles based on four boats
At any time at least one of them is at sea, on patrol, somewhere in the world and has been since the system came online in 1994
It is operated by the Royal Navy and based at the Clyde Naval Base on the west coast of Scotland
The name Trident comes from the American-built UGM-133 Trident II missiles which are carried by the boats
Trident replaced a system of Polaris nuclear weapons based on Resolution-class submarines
What needs to be replaced?
The four submarines carrying the Trident missiles. They are designed to function until the mid 2020s.
How much would a replacement cost?
An estimated £17.5 billion to £23.4 billion.
How much will the new system cost to run?
It is expected to cost around 5% of the annual Ministry of Defence budget - similar to today. Trident opponents, such as the CND, claim replacing the system will cost £100 billion over its lifetime.
Why can't Britain just disarm and give up its nuclear weapons?
The Government insists unilateral nuclear disarmament would leave Britain vulnerable in a dangerous world
The number of warheads has been reduced and is already the smallest stockpile of the five recognised nuclear powers - Britain, the US, Russia, France and China
Government policy is to put British disarmament up for negotiation when other powers have reduced stockpiles to similar levels.
Could the Trident system be reduced?
A review of alternatives produced by former Liberal Democrat defence minister Sir Nick Harvey suggested abandoning continuous at-sea patrols and cutting the number of submarines from four to three.
The Conservatives and Labour rejected this as a "part time" deterrent which would risk increasing international tensions every time a sub was put to sea.
Are there any alternative systems or options?
Alternative platforms, including land-based nuclear weapons and missiles that can be fired from an aircraft, were also considered by the review.
Aircraft-based missiles could be more vulnerable to a pre-emptive strike and that it would take much longer to design and build the new weapon
Land-based cruise missiles would limit Britain's ability to strike anywhere in the world without a third-party nation's help, as well as also being expensive to design and produce
All alternatives to a submarine system would still need at least two new submarines to cover the capability gap between the end of the current Trident programme and the new alternative, significantly increasing the cost of all alternatives