According to the UK National Defence Association, the Prime Minister must guarantee that he will increase spending on the armed forces in line with inflation for the full five years of the next parliament.

In an open letter published last week, the association says that the UK must ‘ring-fence’ the Ministry of Defence budget after the next General Election “to retain a credible defence”.

The letter comes ahead of next month’s NATO summit in Newport, which Mr Cameron will chair.

Allen Sykes, one of the authors of the letter, said: “We already ‘ring-fence’ schools, the NHS and international aid.

Unless we also ‘ring-fence’ our increased defence expenditure, we are sending a signal to our opponents and allies that defence is not our priority.”