The Ministry of Defence was criticised on Wednesday for a "gold-standard cock-up" over eight Chinook helicopters that have cost 422 million pounds but have never flown because they can't operate in cloudy weather.

The National Audit Office (NAO) blamed the MoD for failing to act promptly after it discovered in 2001 that the Chinook Mk3 helicopters could not be flown because of software problems, resulting in a shortfall of support for operations in Afghanistan.

"The Chinook Mk3 story re-emphasises the importance of timely decision making, clearly understanding requirements and proceeding purposefully to the solution," NAO head Tim Burr said in a report to parliament.

Eight Chinooks were ordered from Boeing in 1995 at a cost of 259 million pounds for special operations and were delivered to the MoD in 2001, but have remained grounded because they could only be flown in cloudless skies, despite Boeing having fulfilled its contractual obligations.

The MoD's initial solution was to adopt a programme of "fix to field", to make the Mk3 helicopters operational, but the need for air support in Afghanistan meant it switched to a quicker "reversion" project in 2007, which converted the helicopters to a standard similar to those in the Mk2 Chinook fleet.

"The MoD's progress in fielding the Chinook Mk3 helicopters has been protracted," the NAO said in a statement. "As a result, the shortage of helicopters to support operations has been exacerbated.

"Had the MoD been quicker to progress the 'fix to field' project the subsequent reversion project would have been unnecessary."

A simultaneous attempt to upgrade the more basic Mk2 Chinook already in service has backfired because the night vision equipment installed at a cost of 32 million pounds does not work properly.

Edward Leigh, chairman of the Public Accounts Committee to which the NAO reports, said: "The Ministry of Defence's programme to make airworthy the eight Chinook Mk3 helicopters which it acquired in 2001 for special operations work has been a gold-standard cock-up.

"I have described the purchase of these helicopters as 'one of the most incompetent procurements of all time'.

"Today, nearly seven years since they were delivered, the Chinook Mk3s are still languishing in climate-controlled hangers - despite the fact that they are desperately needed on operations in Afghanistan."

The MoD said it acted quickly to revert the helicopters, which will be available two years quicker than under the "fix to field" programme.

Defence Minister Baroness Ann Taylor said: "The project remains on track in terms of time and budget.

"The Chinook is the most capable support helicopter in Afghanistan. Based on operational need, the reversion project will allow delivery of more Chinooks to theatre in the shortest time-frame."

The helicopter is expected to be delivered by 2009/10.