The government is to spend another £90m rectifying software and avionics problems that have grounded a £259m fleet of eight brand-new Chinook helicopters for nine years.

The Ministry of Defence (MoD) announced before Christmas that it is to boost support for operations in Iraq and Afghanistan, including a £62m contract with Boeing to upgrade helicopters to be sent to the region.

Defence secretary Des Browne described the moves as “measures to make more helicopters available to operations as quickly as possible”. And the MoD said the extra aircraft would “deliver a significant boost to the UK’s operational heavy-lift helicopter fleet”.

But no mention was made of the fact that the eight Boeing-supplied choppers have already cost the taxpayer £259m and were originally to be available for service in November 1998. And despite the huge bill, the Chinooks will still not be ready until 2009.

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”.

As a result it was “not possible to demonstrate that the helicopter’s flight instruments meet the required UK Defence Standards”. So, instead of providing heavy-lift capacity to British troops, the choppers spent eight years in hangars at RAF Odiham.

In 2005, the MoD accounts noted a loss of £205m against the purchase, noting that “while the terms of the contract were met, the helicopters did not meet the operational requirement and could not acquire Military Aircraft Release”. At one point there was even the possibility of “cannibalising” them for spares.

The design work for the £62m conversions will take place in Philadelphia and modification work will then be carried out by UK defence manufacturer QinetiQ at Boscombe Down.

According to a spokesman for the MoD, other options for delivering heavy-lift helicopter support by 2009 were considered and detailed audits show the Boeing deal is value for money.

“The cost is consistent with other modification programmes of this scale,” he said.