Astronauts will climb aboard Discovery next Thursday for a practice countdown

A formal launch date will be set next week, but Nasa officials said Discovery could take off as early as 1 July.

Space shuttle Columbia disintegrated on re-entry three years ago, killing all seven astronauts.

A chunk of foam insulation fell off its fuel tank during launch, hitting a wing and damaging the heat shield.

After changes were made, Discovery was launched in July 2005. But a large foam chunk dropped off the fuel tank again, although it did not strike the orbiter.

To solve the problem, engineers proposed removing foam insulation designed to provide a windshield for fuel pressure lines and electrical cabling along the outside of the fuel tank.

After almost a year of testing and analysis, managers have decided the shuttle is safe to fly with the modifications.

"The tests proved that the cable trays and pressurisation lines are aerodynamically capable of withstanding the launch," said Kyle Herring, a spokesman at the Johnson Space Center in Houston, Texas.

"This is a big one because it certifies the tank to fly with this major change."

Discovery's launch window extends from 1 July to 19 July. A final launch date will be set next week after the completion of a two-day flight readiness review.