The armed forces are facing "critical shortfalls" in air transport and air-to-air refuelling capabilities because of delays in delivering new aircraft, the Whitehall spending watchdog has warned.
The Ministry of Defence has already spent £787 million trying to plug the gap caused by the delays to the Future Strategic Tanker Aircraft (FSTA) and A400M transport aircraft programmes, the National Audit Office (NAO) disclosed.
It said that officials were now looking at the possibility of extending the life of its elderly VC10s - which were supposed to be retired from service in March - by "a few months" in order to eke out as much refuelling capacity as possible.
Once the VC10s have gone, the RAF will be forced to rely on another ageing aircraft - the Tristar, which goes out of service next year - to meet its air-to-air refuelling requirement until the FSTA is ready to take over.
Although the first of the planned 14 FSTAs has now entered service, fuel leaks and fuelling basket "instability" have meant it has had to be restricted to transport duties until the technical problems have been resolved.
Meanwhile, delays to the A400M - which is finally due to enter service in March 2015, six years later than planned - have meant the MoD has had to acquire two additional C-17 strategic transport aircraft and two BAe 146s, for transport and communications, to ease the pressure on the RAF's transport fleet.
Overall, the NAO said there would be some "critical gaps" in air-to-air refuelling and tactical air transport until 2017, and a one-third shortfall in the MoD's stated requirement for tactical air transport aircraft after 2022.
The NAO warned: "The delays in introducing the new aircraft and budgetary constraints have caused critical shortfalls in some capability areas. This is particularly apparent up to the end of UK combat operations in Afghanistan in 2014, when both air transport and refuelling aircraft will be extremely busy, but also from 2022 for air transport, when the Hercules C130J aircraft goes out of service early."
In its annual report on the MoD's major equipment projects, the NAO also disclosed that delays to the £32 million Falcon communications system, which was developed for use in Afghanistan, meant it would not now be ready until after British troops have withdrawn in 2014.
Amyas Morse, head of the NAO, said: "The Ministry of Defence faces a difficult task striking a balance between delivering the capabilities it wants and those it can afford. The continuing problems highlighted in my report show that, if it is to make the most of the money available, the department has more to do to address its long-standing issues on project performance."