“The government needs a safe space away from the public gaze to allow it to consider policy options for delivering the deterrent in the most cost-effective way, unfettered from public comment about the affordability of particular policy options,” it said in response to a freedom of information request from Reuters.

In its strategic defence and security review at the end of last year, the government announced an increase from £25bn in the estimated cost of four new Trident submarines to £31bn, with an additional £10bn to cover overspends.