The RAF’s next-generation drone programme is threatened by pilot shortages and a cost overrun of £325 million, the Ministry of Defence has admitted.

Problems with the plan to buy 16 American-made Protector drones, which can fly for twice as long as the present Reaper model, were revealed in a letter to MPs. Sir Stephen Lovegrove, the MoD permanent secretary, told the Commons public accounts committee that the cost of the unmanned devices had overrun by 40 per cent and would top £1.1 billion.

Problems with training and retaining enough drone operators were revealed as a threat to the programme, which is running more than two years late. “The most significant risk is the RAF’s ability to generate and sustain the volume of trained personnel