A £3.2 billion programme to contract out the training of military pilots is running almost six years behind schedule, the Whitehall spending watchdog has found.
Ascent - a consortium of Lockheed Martin and Babcock - was originally awarded the 25-year contract to train aircrew for the Ministry of Defence (MoD) in 2008 in a move intended to reduce costs and training times.
The scheme was scheduled to be running at full capacity by 2014, but the National Audit Office (NAO) said that a series of delays meant it would not now get there until the end of 2019.
It found the early years of the contract were dogged by "significant concerns" about Ascent's performance but while the MoD has so far paid out more than £143 million, it has only been able to claw back £308,000 in deductions for its failure to meet its responsibilities.
The NAO said the delays were in part due to the "substantial" cuts in the numbers of aircrew undergoing training, which had seen the value of the contract slashed from £6.8 billion to £3.2 billion.
While Ascent's performance was considered to have improved since 2012 after the MoD raised its concerns with the shareholders, the NAO said there was still "much to do" if it was to achieve the planned benefits of the contract.
The head of the NAO Amyas Morse said: "The department needs to understand better actual training performance and what affects performance before it can secure significant improvements from Ascent.
"Otherwise, there is a real risk that moving to the new training will affect the military's ability to train the right number of aircrew at the right time."
Defence Procurement Minister Philip Dunne said: "The programme was thoroughly overhauled in 2012 and early implementation issues are now behind us.
"We are now on a much firmer footing and moving forward to deliver flight training for aircrew, more rapidly and efficiently, with greater flexibility to adapt to new platforms."