An error has been identified in the written answer given on 23 October 2015.
The correct answer should have been:
The 2014 Update to ParliamentThe May 2011 Initial Gate Parliamentary Report set out an estimate for the Successor submarine acquisition of around £25 billion, based on a four boat solution, spread over some 25 years. These estimates are currently being refreshed to inform the Comprehensive Spending Review and Strategic Defence and Security Review.
Once the new fleet of SSBNs come into service, we expect that the in-service costs of the UK’s nuclear deterrent, which include the costs of the Atomic Weapons Establishment, basing and disposals, will be similar to the current system, at around six per cent of the defence budget.
While we have no plans to replace the current Trident D5 missile, we are participating with our US partners in a programme to extend the current life to the 2060s. The estimated cost is around £250 million.