Defence chiefs must find nearly £6 billion of additional savings or officials will be forced to cut or delay plans for new tanks and aircraft, the spending watchdog has warned.

The National Audit Office said the risks to the affordability of the Ministry of Defence's 10-year spending plan were greater than at any point since its introduction in 2012.

The sharp fall in the value of the pound since Britain voted to leave the European Union has made the situation worse because some weapons projects are paid for in dollars and euros.

The NAO warned that “changes in foreign exchange rates, such as those that happened after the EU referendum, can pose a significant risk” to the “affordability” of weapons’ projects.

The projected cost of funding the MoD's schemes rose to £178 billion last year - an increase of 7 per cent compared with a rise of just 1.2 per cent between 2013 and 2015.

A large part of the increase was due to the £24.4 billion of additional commitments announced in the 2015 Strategic Defence and Security Review.

Meeting the cost will absorb all of the £10.7 billion “headroom” built into the plan to meet emerging new requirements and require additional savings of £5.6 billion over the next 10 years.