The UK's Trident nuclear arsenal is to get a £1bn boost over three years to ensure it is "reliable and safe" for its remaining two decades of service.
The issue of Trident was controversial from the outset
Defence Secretary John Reid said cash would go to updating the Atomic Weapons Establishment at Aldermaston, Berks.
The investment will ensure the UK can "maintain the existing Trident warhead stockpile" Dr Reid told MPs.
Tony Blair has said Britain wants to retain its independent nuclear deterrent after Trident.
But the prime minister has also stressed no decision has been taken about how to replace the current system.
In a Commons written answer, Dr Reid said: "In the absence of the ability to undertake live nuclear testing - given that the UK has signed and ratified the Comprehensive Test Ban Treaty - it is necessary to invest in the facilities at AWE [Aldermaston] which will provide assurance that the existing Trident warhead stockpile is reliable and safe."
Critics say Trident is outdated, designed to deal with the threat posed by the Soviet Union during the Cold War, and should now be stood down.
Last month Mr Blair told Labour MP and ex-minister Chris Mullin it was too early to rule in or rule out any particular option about what to do in terms of replacing Trident.
"As we set out in our manifesto, we're committed to retaining the UK's independent nuclear deterrent but I'm sure there will be plenty of opportunities to discuss this before the final decision is taken."