THE RAF is facing "critical shortfalls" in air transport and air-to-air refuelling capabilities because of delays in delivering new aircraft to RAF Brize Norton, the Government spending watchdog warned today.

The Ministry of Defence has already spent £787m trying to plug the gap caused by the delays to the Voyager refuelling plane and Airbus A400M Atlas transport aircraft programmes, the National Audit Office (NAO) said.

In its annual report on the MoD's major equipment projects, the NAO said that MoD officials were now looking at the possibility of extending the life of the West Oxfordshire air station's fleet of elderly VC10 refuelling aircraft, which are 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have gone, the RAF will be forced to rely on another ageing former airliner, the Tristar - which is scheduled to be withdrawn by the RAF next year - to meet its air-to-air refuelling requirement until the new Voyager refuelling tankers, converted from Airbus A330-200 airliners, are ready to take over.

Although the first of the 14 Voyagers due to be based at Brize Norton is now in service, fuel leaks and fuelling basket "instability" during air-to-air refuelling trials with Typhoon and Tornado attack jets have meant the plane is restricted to transport duties until the technical problems have been resolved.

Meanwhile delays to the A400M Atlas, now expected to go into service at Brize Norton in March 2015, six years later than planned, meant the MoD has had to acquire two extra Boeing C-17 Globemaster heavy transport aircraft and two BAe 146s jets for transport and communications, to ease the pressure on the base's transport fleet.

Overall, the NAO said there would be some "critical gaps" in the RAF's 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, when the Lockheed Hercules C130J planes based at Brize Norton are scheduled to be retired.

"The delays in introducing the new aircraft and budgetary constraints have caused critical shortfalls in some capability areas," the NAO warned.

"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."

The NAO found that cost projected overruns for the 16 biggest MoD programmes had increased by £468m over the past year, while total delays have risen by 139 months.

Defence Secretary Philip Hammond said: "Thanks to rigorous financial management, this Government has dramatically reduced the annual cost growth of the biggest equipment projects from more than £3bn under the last year of the previous administration, to under half-a-billion pounds."

But Labour MP Margaret Hodge, the chairman of the House of Commons public accounts committee, said: "Yet again, the MoD's annual report card on its 16 largest defence projects has found unacceptable cost increases and delays, making it harder for the ministry to deliver today's capability needs and close tomorrow's capability gaps."