The US space agency (Nasa) has failed to meet tough safety recommendations issued after the Columbia shuttle break-up in 2003, experts say.
Nasa aims to launch Discovery as early as 13 July
The independent Stafford-Covey Commission said a risk remained that pieces of foam and ice could break off and hit the shuttle at lift-off.
It also said the orbiter had not been sufficiently hardened and it lacked an in-flight repair system.
The findings could affect Nasa's plans to resume shuttle flights in July.
Nasa has been aiming to launch shuttle Discovery as early as 13 July.
Key decision due
The findings of the 26-member panel came after a deliberate and prolonged discussion.
The experts said that overall Nasa had complied with 12 out of 15 recommendations laid down by the Columbia Accident Investigation Board and considered essential for the resumption of shuttle flights.
Commission members went so far as to praise the space agency for the work it had done in going beyond the basic requirements of some recommendations.
The panel's head, Richard Covey, said the three outstanding recommendations were the most technically challenging.
As well as concerns about debris striking the shuttle, the commission said Nasa had not sufficiently "hardened" the shuttle to protect against damage, and had not designed an in-flight repair system to deal with serious incidents.
When pressed by reporters to answer if it was safe to launch Discovery in July, Mr Covey, a former astronaut himself, declined to answer specifically.
However, he said that if he were younger he would have no concern about flying on the shuttle.
The panel members stressed that the final decision on whether it was safe to launch the shuttle rested with Nasa.
Top agency officials concluded last week that ice striking the orbiter's heat shield remained a possibility, but that the risk was low enough to safely resume flights.
"At the end of the day, the recommendation... was that we're in an acceptable risk posture and that we proceed on with the launch," shuttle programme manager Bill Parsons said.
Nasa begins a two-day Flight Readiness Review on Wednesday. The outcome of this meeting will determine whether Discovery flies in July.
A piece of insulating foam came off Columbia's external fuel tank during lift-off in January 2003, leaving a hole in the left wing.
The shuttle disintegrated during re-entry over Texas on 1 February 2003, killing all seven of its crew.