Weeks away from its first space shuttle launch since the Columbia disaster, Nasa has still not met three crucial safety recommendations put in place after the accident, an advisory panel to the agency has warned.

The independent Return to Flight Task Group, appointed in 2003 to monitor Nasa's work to improve the safety of its shuttle fleet, said that the agency has not yet eliminated the possibility of pieces of ice and foam breaking away from the shuttle at lift-off and damaging the space craft.

The 26-member group, which is led by two retired astronauts, also warned yesterday that Nasa still does not fully understand the damage ice can do to the shuttle and that the agency's plans for making repairs to the space craft while in orbit "fell short".

It is unclear whether the findings will delay the launch of the Discovery on July 13. The Discovery is undertaking the first shuttle mission since the Columbia broke up at high altitude above Texas on February 1 2003.

The task force, which has a purely advisory role, will present a summary of its report to Nasa today ahead of the agency's official "flight readiness review" later this week.

The Nasa Administrator, Michael Griffin, said yesterday that the embarrassing findings of the taskforce would contribute to "healthy debate" over the coming days.

"The independent Return to Flight Task Group... has performed a valuable public service," Dr Griffin said. "As an engineer, I know that a vigorous discussion of these complex issues can make us smarter. I anticipate, and expect, a healthy debate in our upcoming Flight Readiness Review."

The advisory group's concerns became clear at its final public meeting. The group praised Nasa for its hard work and for making 12 out of the 15 safety improvements recommended after the Columbia disaster, but warned that the three most technically difficult recommendations had not been met.

Doubts still hang over whether Nasa can control the risk of pieces of foam and ice breaking away from the shuttle on take-off. A piece of foam the size of a suitcase fell away from the external fuel tank of the Columbia and caused damage to its left wing, which led to the shuttle's disintegration as it re-entered the earth's atmosphere.

The task force acknowledged that the Nasa safety team has made changes to the shuttle's design to reduce the amount of foam and ice that can fall away, but they warned that the problem has not been solved.

"Although the program has performed an extensive effort to reduce debris for return to flight, there is still the potential for foam and ice to cause damage to the orbiter that exceeds safe entry limits," the group said.

Colonel James C. Adamson, a former astronaut on the panel, reported that the plans for mid-orbit repairs to the shuttle were also unsatisfactory.

Nasa has developed five techniques to repair the shuttle in flight, but Colonel Adamson said: "We found that Nasa fell short of meeting that recommendation, although they had put forth a yeoman's effort in coming up with all of the options that they could conceive of for repair."

Another task force member Joseph Cuzzupoli, a Kistler Aerospace Corporation vice president, said that Nasa had tackled the problem of ice damage to the shuttle too late, studying it in detail only in the last few months.

Pieces of ice gather on the external fuel tanks of the shuttle, which are filled with super-chilled fuel, and can then break away at blast-off. Concerns over ice damage pushed the Discovery's initial launch date from May to its current date in July.

"Foam is characterised pretty good," Mr Cuzzupoli told reporters. "The ice story is still coming together."

The task force also noted that Nasa has also put off long-term changes to the design of the shuttle because of the fleet’s planned retirement in 2010.

The chairman of the advisory group, Richard Covey, refused to give a definitive answer to the question of whether the Discovery was safe to take-off, but said that if he was still on flight duty as an astronaut, he would take the risk.

"Quite honestly, we were trying to stay away from being someone who gave a thumbs-up or a thumbs-down on whether it’s OK to fly. We weren’t going to do that," said Mr Covey, who made four flights on space shuttles. "Is it a miss if you’re at 95 per cent?"

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