To ask the Secretary of State for Defence what factors were taken into account in reaching the conclusion in the White Paper on the future of the UK's nuclear deterrent that the upper limit on the number of operationally available warheads should be 160; and what (a) methodology and (b) calculations were used in reaching that conclusion. (119285)

As we said in paragraph 4-9 of the White Paper ‘The Future of the United Kingdom's Nuclear Deterrent’, Official Report, column 6994, published on 4 December, in determining the scale of minimum nuclear deterrent,

“We need to make a judgment on the minimum destructive capability necessary to provide an effective deterrent posture. This judgment requires an assessment of the decision-making processes of future potential aggressors, and an analysis of the effectiveness of the defensive measures that they might employ.”

The new upper limit of less than 160 operationally available warheads reflects the results of that judgment, together with our NATO commitments, and the way in which we operate the deterrent submarines. I am withholding further details as their disclosure would, or would be likely to, prejudice the defence of the United Kingdom.