However, Cullen said the MoD’s insistence on using the “early 2030s” formulation meant it was impossible to tell the exact impact of the delay.
The four existing Trident submarines were originally built to last 25 years, meaning they would have been out of service between 2017 and 2024. Their life has been guaranteed by a series of extensions, but they will have to last well over 30 years.
Two functioning submarines – one at sea and one ready to replace it – are the minimum necessary to maintain the continuous nuclear deterrent. Last year it emerged that two of the four vessels were out of action at the same time, one of which, HMS Vanguard, is still having its nuclear reactor refuelled at a cost of £200m.