Shadow Defence Secretary Iain Duncan Smith has called for the Ministry of Defence to reopen its inquiry into a helicopter crash on the Mull of Kintyre, which killed 29 people.

Mr Duncan Smith's demand, which has been backed by Liberal Democrat spokesman Menzies Campbell, came after a new report said there was an "unquantifiable risk" associated with the software on board the RAF Chinook, which crashed in June 1994.

The report by the National Audit Office (NAO) into the RAF's fleet of Chinook twin-rotor helicopters quoted Ministry of Defence experts as saying the safety-critical software was "not fit for purpose".

The report, published on Friday, says the MoD team found the engine-control FADEC software to be "unverifiable".

The Chinook was carrying some of Britain's top security experts when it crashed.

'Negligence' finding

An RAF inquiry subsequently returned a verdict of "gross negligence" against the two dead pilots, Flight Lieutenants Jonathan Tapper and Rick Cook, who were accused of calculating a wrong rate of climb.

But their families have long pressed for a new inquiry, claiming it would prove the software was to blame for the wrong rate of climb.

Despite claims the FADEC system software was unsafe, the government has continually refused to reopen the inquiry because of a "lack of new evidence".

Mr Duncan Smith said: "The NAO report clearly highlights the need for an independent inquiry into the Mull of Kintyre Chinook helicopter crash, which I have been calling for for some time."

He said: "We believe in the interests of fairness, to stop all further speculation and, above all for the peace of mind of the families who died and of the servicemen now piloting the Chinook helicopter, it is necessary to clear the air on this issue once and for all."

Mr Campbell said the new report "added more weight" to calls for a new inquiry.

He said: "The information that the Ministry of Defence approved a fleet of Chinooks, despite the Defence Evaluation and Research Agency's warning of the 'unquantifiable risk' associated with on-board software, is profoundly disturbing.

"In the light of this evidence the government now owes it to the families of the dead pilots to re-examine the case."

The NAO report summarises concerns expressed by the Ministry of Defence's own software specialists at Boscombe Down.

Experts at the Wiltshire facility found that the software in question was "unverifiable" and, in their view, not fit for the purpose.

The government auditors also say that the Boscombe Down experts were critical of the amount of testing to the FADEC system carried out by its manufacturer.

Malfunction 'unlikely'

In October, a House of Lords debate concluded there was no justification for holding a further inquiry.

Defence Minister of State, Baroness Symons of Vernham Dean, said the government was "more than willing" to look at any new evidence but it did not feel there was any reason to reopen the original investigation.

The aircraft was ferrying its passengers to Inverness from Northern Ireland when it came down in thick mist.

The RAF board of inquiry which followed said the likeliest cause, based on the limited evidence, was that the helicopter's rate of climb was wrong and technical malfunction was unlikely, although this could not be positively disproved.