HMS Vanguard: Trident submarines will last until mid 2020s
The UK's continuous seaborne nuclear deterrent is at risk, MPs have warned, amid doubts that a new generation of submarines will be ready on time.
Two of the UK's four Vanguard nuclear subs will leave service in 2024, with work on replacing them under way.
But the Public Accounts Committee said the timetable for replacement was "extremely tight" and the record of past procurement projects was not good.
Ministers said there was no threat to the deterrent, in operation since 1968.
Successive governments since then have remained committed to a continuous nuclear deterrent at sea, requiring at least one nuclear-armed submarine to be on patrol at any one time.
Labour set out its plans to build a replacement fleet of submarines in 2006 when it also gave the go-ahead for the £20bn upgrade of its Trident nuclear missile capability.
The Committee said the Ministry of Defence (MoD) had given itself 17 years to complete the construction of the new submarines when best practice recommended a period of not less than 18.
"The department's timetable for completing the design and build process is extremely tight," said its chairman, Conservative MP Edward Leigh.
"The MoD's track record in delivering major defence projects on time is not exemplary," he added.
Vital decisions on design features, including the size of missile compartments, must be made by this autumn, Mr Leigh added.
However, he expressed concerns that the question of the missile component would have to be taken in advance of work on developing a successor to the US Trident missile.
Despite US assurances about its compatibility, the committee was worried that the UK would have no control over the development of the next generation of missile.
"Our programme to have a renewed nuclear deterrent will depend on yet to be taken decisions by the US on the dimensions of the successor missile," he said.
"The MoD is taking steps to reduce the risk of a new missile not fitting in our submarines but there is no guarantee it will."
But the MoD said there was no doubt that missiles and missile components in future submarines would be compatible.
Defence procurement minister Quentin Davies said the UK's ability to maintain the submarine deterrent on a continuous basis was not in doubt.
Gordon Brown said earlier this week that he was prepared to include the UK's nuclear arsenal in multilateral arms control talks.