One of America's most experienced astronauts has denounced the space shuttle as a deathtrap and accused US space officials of stifling all concerns raised about its safety.

The revelation comes as America prepares to commemorate the 20th anniversary of the Challenger disaster. Seven astronauts were killed on 28 January 1986, when their shuttle exploded 73 seconds after take-off.

Veteran astronaut Mike Mullane's outburst therefore comes at a deeply embarrassing time for the Nasa. Apart from dealing with the Challenger anniversary, it is now struggling to save its remaining space shuttles so they can complete the international space station.

However, all three - Discovery, Atlantis and Endeavour - are still grounded because engineers have not yet fixed insulation problems that doomed Challenger's sister craft, Columbia, in 2003. 'It's the most dangerous manned spacecraft ever flown,' said Mullane, who took part on three shuttle missions before retiring in 1990. 'It has no powered-flight escape system... Basically the bail-out system we have on the shuttle is the same bail-out system a B-17 bomber pilot had in World War II.'

It was this lack of ejector seats that ensured the deaths of Challenger's astronauts. Such a powered escape system could have blasted them from their stricken ship and saved them.

'That was the true tragedy of Challenger. Nothing was learnt. Only janitors and cafeteria workers at Nasa were blameless in the deaths of the Challenger seven,' said Mullane. 'Columbia was a repeat of Challenger, where people had a known design problem and launched anyway.' Mullane added that astronauts deserved some share of responsibility for not pursuing safety issues more doggedly.

It is estimated that it now costs Nasa $5bn a year to pay for the 16,000 engineers who maintain the fleet - even if none of them actually flies. As a result, the bill for designing, building and launching the shuttle has now topped $150 billion.

Engineers point out that the craft has been responsible for putting the Hubble space telescope and the International Space Station into orbit. However, each craft has thousands critical components. A failure of any of these will doom a craft. On Challenger, it was a seal on a solid rocket booster. On Columbia, it was piece of loose insulation foam.

'You walk in terrified of doing anything that might jeopardise your one chance to get to space,' Mullane said in an interview to promote his new book, Riding Rockets. 'It's not like other jobs, where if you get frustrated you can go in to your boss and say "Shove it!" You can't do that at Nasa because there's no other place to go fly shuttles.'

The three remaining shuttles are supposed to retire in 2010 after completing construction work on the orbiting space station, which has been operating with a skeleton crew of two since the Columbia disaster.