In July 2016 Britain's MPs voted to back the renewal of the UK's Trident nuclear weapons system by 472 votes to 117.
Here's a full breakdown of the situation as it stands.
WHAT WE'VE GOT NOW
The Royal Navy's Trident fleet consists of four ballistic missile submarines (SSBNs)
Vanguard, Victorious, Vigilant and Vengeance.
The Trident fleet always has one armed submarine at sea, one undergoing maintenance, and two in port or on training.
When armed, the boat carries eight missiles with 40 warheads each.
The 150 metre (429 feet)-long Vanguard-class submarines were built with a 25-year life expectancy, taking them into the 2020s.
COST OF THE CURRENT PROGRAMME
When the current programme was initially paid for in the 1990s, it cost £9.8 billion.
Average capital and running costs were estimated at £1.2 to £1.7 billion in 2005-06.
That represented 3-4.5% per cent of the annual defence budget in 1994, and was projected to rise to 5.5% by 2007-08.
COST OF THE REPLACEMENT PROGRAMME
An artist's impression of the Dreadnought-class submarine
Of the Trident programme's three components - the submarines, missiles, and warheads, it will be the Vanguard-class subs that will become obsolete the soonest.
The government has estimated that the Dreadnought-class subs, in what is previously known as the Successor programme (to replace the Vanguard-class) will cost £31 billion for their entire 30-year lifetime (this includes adjustments for inflation over that time).
That works out to 6% of defence spending annually, at the current rate.
In addition, the government has recommended that a further £10 billion be set aside as a "contingency".
Meanwhile, Greenpeace claims that the cost will run closer to £34 billion.
A HISTORICAL PERSPECTIVE
HMS Repulse, which carried Polaris missiles, in the Firth of Clyde, 1979
Before Trident, the UK agreed a deal with the US in 1962 to acquire the Polaris submarine-launched ballistic missile system. The four boats built to carry the missiles, Resolution, Renown, Repulse and Revenge formed the basis of Britain's nuclear deterrent.
Each sub reportedly cost £35 million, and each missile £350,000, with the total cost around £300 million.
A stolen sub with Polaris missiles featured in the James Bond film The Spy Who Loved Me (1977)