The latest NAO report into the MOD's efforts to make eight Chinook helicopters ready for action said the costs have rocketed to £500m.

Thirteen years after they were ordered, the helicopters remain grounded in an air-conditioned hangar.

In January this year, Computing exclusively reported that the software upgrade required to make the £259m fleet of Chinooks air worthy would cost £90m.

"According to a critical Commons Defence Committee report in December 2006, the fleet was grounded because the contract with Boeing “did not specify that software documentation and codes for avionics systems should be analysed in accordance with UK Defence Standards”" Computing reported at the time.

The latest report from the National Audit Office said: "The total costs associated with the Chinook Mk3s from their initial procurement through to completion of the reversion project will be £422m, not including further expenditure required to sustain the night enhancement package necessary to conduct demanding very low-light missions."

Tim Burr, head of the National Audit Office, said: "Since 2001 the MOD has been trying to work out how to make the Chinook Mk3 helicopters available for operations. Its original Fix to Field project progressed very slowly. In 2007 changing operational imperatives meant that the Department decided to start a new reversion project to make the helicopters operational more quickly, though with a lesser capability. The Chinook Mk3 story re-emphasises the importance of timely decision making, clearly understanding requirements and proceeding purposefully to the solution."