HMS Vanguard: Trident submarines will last until mid 2020s
Critical decisions must be taken urgently on replacing Britain's nuclear weapons system if it is to be ready in time, says the National Audit Office.
The spending watchdog warned of a "challenging timetable" if the new submarines were to be ready before the old Vanguard class was retired in 2024.
Key developments would have to be made by September 2009, if the project is to remain on course, it said.
Ministers agreed it was challenging but said the project was on schedule.
The Trident system - made up of submarines, missiles and warheads - are due to end their working lives in the 2020s.
Former prime minister Tony Blair gave the go-ahead to replace the system in 2006, which was estimated at the time to cost between £15bn and £20bn.
The National Audit Office said "good progress" had been made so far, but that 17 years were required to develop a new submarine, which left no room for manoeuvre if the 2024 target date was to be met.
Factors which could throw the project off timetable or off budget included skills shortages in building submarines, uncertainty over inflation, VAT and contingencies, and a reliance on monopolies in submarine manufacturing.
TRIDENT MISSILE SYSTEM
Missile length: 44ft (13m)
Weight: 130,000lb (58,500kg)
Diameter: 74 inches (1.9m)
Range: More than 4,600 miles (7,400km)
Power plant: Three stage solid propellant rocket
Cost: £16.8m ($29.1m) per missile
Source: Federation of American Scientists
"There is a challenging timetable to meet if continuous at-sea deterrence is to be maintained," the report said.
"There is currently little scope for incorporating time contingency in the overall programme to deal with slippage in any of these areas."
The National Audit Office said key developments had to be in place by September 2009 - including establishing exactly how long the Vanguard submarines' lives could be extended and "robust" estimates of costs of the new system over its lifetime.
NAO head Tim Burr said "much groundwork" had been done since 2006 but added: "Critical decisions about the design of the future submarine class and the commercial strategy required to incentivise monopoly suppliers now need to be taken so that the Ministry of Defence can stick to its demanding schedule and assure value for money over the life of the programme."
Shadow defence secretary Liam Fox said the Trident system was Britain's "best protection from nuclear blackmail overseas".
He said: "Labour's failure to make decisions on this vital issue now potentially puts our national security at risk.
"This is a failure of the first duty of government, which is the defence of the realm."
Defence Secretary John Hutton accepted getting the project finished on time and on budget would be a challenge.
But he said: "The UK future nuclear deterrent programme is on schedule with the current emphasis firmly on a new class of submarines to replace the current Vanguard class.
"The first replacement submarine is planned to be in service in around 2024.
"We are fully aware of the timetabling risks identified by the NAO and the need to manage them. Meeting the in-service date is a challenge, but every avenue to reduce both time and cost is being pursued."