The two-month delay occurred because managers decided the spaceship's fuel tank, which triggered the 2003 Columbia disaster, is not quite ready to fly.

Nasa first set the launch date for 15 May, but it has been pushed back twice.

The shuttle fleet has been grounded since the 2003 accident, which killed seven astronauts.

Driving Nasa's decision to postpone the flight to July - the next possible launch period - were lingering concerns about accumulations of ice forming on the outside of the external tank. The fear is the ice could come away as the orbiter blasted off the launch pad and strike delicate parts of the ship's thermal shield.

Torn apart

Columbia was struck by a piece of lightweight foam insulation that fell off the fuel tank during launch. Far lighter than ice, the foam gouged a hole in the ship's wing, which was torn apart as the shuttle flew through the atmosphere 16 days later for landing.

Nasa redesigned the tank and replaced the wedge of foam that slipped off Columbia's tank with new electric heaters - but problems remain.

Last week, shuttle programme managers told reporters that engineers had assessed 170 sources of potential debris and cleared all but a handful as a potential threat to shuttle safety. A follow-up review was held at the Kennedy Space Center on Tuesday and Wednesday, but managers could not dispel their concerns.

One worry is a 21m-long (70ft) propellant line running along the outside of the tank.

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Before launch, the tank is filled with 1,900,000 litres (500,000 US gallons; 420,000 imperial gallons) of liquid oxygen and liquid hydrogen, both of which need to remain hundreds of degrees below zero despite the Florida sun and humidity. The fuels feed the shuttle's main engines during launch.

Engineers are concerned frost and ice could build up near the top of the propellant line and break off during launch, damaging the shuttle.

Nasa may decide to add an electric heater to the suspect area before clearing Discovery for flight. The foam insulation around the line already has been revamped to prevent condensation from pooling and freezing into chunks of ice. However, when the modified tank was filled with fuels during a launch pad test two weeks ago, the potential for ice formation remained.

"We will not launch if we think there is a concern for an unacceptable amount of ice to hit the orbiter," deputy shuttle programme manager, Wayne Hale, told reporters during the teleconference last week.

New tank

Nasa also could decide to replace Discovery's tank with one already outfitted with the new heater. That tank was originally intended to fly on a later mission.

Because of new flight rules imposed after the Columbia accident, Nasa has far fewer suitable periods of time for launching the shuttle. For now, lift-off, as well as the separation of the shuttle fuel tank just before the ship reaches orbit, must occur during daylight so that cameras will have good lighting for still pictures and video.

Nasa had hoped to have its first two post-Columbia missions complete by summer's end so that managers could look into relaxing the launch restrictions and return full-steam to space station construction and crew rotation flights

The new Nasa administrator Michael Griffin also has said the shuttle must safely return to flight before he will consider re-scheduling a servicing mission to the Hubble Space Telescope that was cancelled following the Columbia accident.

Griffin's predecessor, Sean O'Keefe, made that call to assure that shuttle astronauts would be able to seek shelter aboard the space station should their ship become too damaged to safely return to Earth.

Even if Nasa had realised the extent of the damage to Columbia's wing, the crew had no option to reach the station because the shuttle was on a free-flying research mission without the means to shift orbit. Likewise, a crew flying to Hubble would not be able to reach the outpost.