Question to the Ministry of Defence:

To ask the Secretary of State for Defence, for what reasons the cost estimates of the Successor class of submarines in the Strategic Defence and Security Review 2015 differs from his Department's previous cost estimates for those submarines.

Designing and building submarines is one of the largest programmes and one of the most complex activities that the Ministry of Defence and UK Industry has ever undertaken. It is the purpose of a design phase to improve our understanding of costs and timescales, which we have now done. The current estimates reflect what we have learned since the design phase began.

As stated in the Strategic Defence and Security Review (SDSR) 2015 (Cm9161), our latest estimate of the total cost to manufacture the four Successor submarines reflects greater maturity of the design and understanding of the supply chain, amounts to £31 billion; we will also set a contingency of £10 billion. This level of contingency represents about 35% of the costs to completion and is a prudent estimate based on past experience of large, complex projects.