A report from the National Audit Office, the public spending watchdog, found the price of upgrading the Navy's bases and yards to house the submarines had risen by pounds 800m or 72 per cent.
Much of the responsibility for the record-breaking final bill rests with the Ministry of Defence and the Property Services Agency for poor management and lack of planning.
Bases at Faslane and Coulport on the Clyde have been expanded to take the new Trident submarines, at 16,000 tonnes and 150m (492ft) long much bigger than the Resolution and Polaris class vessels they replace.
The NAO study says the works comprised 110 individual projects, some of which were enormous: a new lift to raise submarines is as high as an 11-storey building and as long as Wembley Stadium; a jetty handling explosives is as tall as Nelson's Column and as long as two football pitches.
The projects were due to be finished in July 1992 but overran by 12 months.
One crucial reason for the soaring cost was that the facilities were being redesigned as they were being built. Officials became worried about the potential dangers to the public, especially from an earthquake. At 'a critical point in the programme', notes the NAO, 'additional work was required to demonstrate that the facilities could safely withstand a more serious earthquake'.
The cost of keeping the Polaris vessels and missiles safe during building work was also higher than expected.
A key factor was that despite the work's size and complexity, officials used a standard government contract.
'It assumed that designs were largely complete when contracts were awarded,' says the NAO. 'But this proved not to be the case because, in order to meet the tight timetable, contracts were let well in advance of firm designs being established. To accommodate design changes . . contractors demanded time and payments additional to those negotiated when the contracts were placed. Subsequent design development risk was thus borne by PSA and the department.'
When the budget was drawn up, not enough attention was given to a contingency fund to pay for the design changes. All the budget, including a contingency fund, was swallowed by the end of 1990.