SNP call to scrap nuclear defence plans
The National Audit Office (NAO) said the replacement of Britain's nuclear deterrent on time and within budget presented the government with "considerable challenges".
The public spending watchdog said the Ministry of Defence had no time to spare if it was to have new submarines ready in time to replace the outgoing Vanguard class in 2024.
But it also pointed to a series of obstacles, including skills shortages, uncertainties with the budget and the need to obtain value for money from suppliers.
It was costed by the government at 15-20 billion in 2006-7 prices, although critics predict those figures will soar in the years ahead.
The NAO said "good progress" had been made in the initial stages since work began last year.
But it cautioned that the 2024 target date allowed only the 17 years required to develop a new submarine if the MoD was to avoid the first break in Britain's at-sea nuclear deterrence since it was introduced in 1968.
"There is a challenging timetable to meet if continuous at-sea deterrence is to be maintained," it said.