The Ministry of Defence has reduced the projected cost of its 20 largest weapons systems by £699m in the past year, ending a two-year slide in which its biggest arms programmes ballooned by £4.8bn.

Although details of the cost reductions will not be released until the National Audit Office sets out its annual analysis of the MoD's arms programmes on Friday, MoD officials said by far the biggest saving came from the Joint Strike Fighter aircraft, where the department cut £659m to take its projected cost to £1.9bn.