In a letter to the White House, Don Nelson, who served with Nasa for 36 years until he retired in 1999, wrote to President George W. Bush warning that his 'intervention' was necessary to 'prevent another catastrophic space shuttle accident'.

During his last 11 years at Nasa, Nelson served as a mission operations evaluator for proposed advanced space transportation projects. He was on the initial design team for the space shuttle. He participated in every shuttle upgrade until his retirement.

Listing a series of mishaps with shuttle missions since 1999, Nelson warned in his letter that Nasa management and the Aerospace Safety Advisory Panel have failed to respond to the growing warning signs of another shuttle accident. Since 1999 the vehicle had experienced a number of potentially disastrous problems:

· 1999 - Columbia's launch was delayed by a hydrogen leak and Discovery was grounded with damaged wiring, contaminated engine and dented fuel line;

· January 2000 - Endeavor was delayed because of wiring and computer failures;

· August 2000 - inspection of Columbia revealed 3,500 defects in wiring;

· October 2000 - the 100th flight of the shuttle was delayed because of a misplaced safety pin and concerns with the external tank;

· April 2002 - a hydrogen leak forced the cancellation of the Atlantis flight;

· July 2002 - the inspector general reported that the shuttle safety programme was not properly managed;

· August 2002 - the shuttle launch system was grounded after fuel line cracks were discovered.

White House officials rejected Nelson's plea for a moratorium. He tried to talk again to Nasa's administration about his worries in October but was again rebuffed.

Yesterday Nelson told The Observer that he feared the Columbia disaster was the culmination of 'disastrous mismanagement' by Nasa's most senior officials and would inevitably lead to the moratorium he was calling for.

'I became concerned about safety issues in Nasa after Challenger. I think what happened is that very slowly over the years Nasa's culture of safety became eroded.

'But when I tried to raise my concerns with Nasa's new administrator, I received two reprimands for not going through the proper channels, which discouraged other people from coming forward with their concerns. When it came to an argument between a middle-ranking engineer and the astronauts and administration, guess who won.

'One of my biggest complaints has been that we should have been looking for ways to develop crew escape modules, which Nasa has constantly rejected.'

His claims emerged against a background of growing concern over the management of safety issues by Nasa.

They followed similar warnings last April by the former chairman of the Aerospace Safety Advisory panel, Richard Bloomberg, who said: 'In all of the years of my involvement, I have never been as concerned for space shuttle safety as now.'

Bloomberg blamed the deferral or elimination of planned safety upgrades, a diminished workforce as a result of hiring freezes, and an ageing infrastructure for the advisory panel's findings.

His warning echoed earlier concern about key shuttle safety issues. In September 2001 at a Senate hearing into shuttle safety, senators and independent experts warned that budget and management problems were putting astronauts lives at risk. At the centre of concern were claims that a budget overspend of almost $5 billion (£3bn) had led to a culture in Nasa whereby senior managers treated shuttle safety upgrades as optional.

Among those who spoke out were Democratic Senator Bill Nelson of Florida, who warned: 'I fear that if we don't provide the space shuttle programme with the resources it needs for safety upgrades, our country is going to pay a price we can't bear.

'We're starving Nasa's shuttle budget and thus greatly increasing the chance of a catastrophic loss.'

Although Nasa officials said that improvements were being made they admitted that more needed to be done.

A year earlier, a General Accounting Office report had warned that the loss of experienced engineers and technicians in the space shuttle programme was threatening the safety of future missions just as Nasa was preparing to increase its annual number of launches to build the International Space Station.

The GAO cited internal Nasa documents showing 'workforce reductions are jeopardising Nasa's ability to safely support the shuttle's planned flight rate'.

Space agency officials discovered in late 1999 that many employees didn't have the necessary skills to properly manage avionics, mechanical engineering and computer systems, according to the GAO report.