Weather was a persistent worry leading up to Saturday's scheduled launch, with storm clouds hovering over Kennedy Space Centre.

The forecast for Sunday is no better, with a 60% chance that storms could force a delay of that launch too.

Mike Leinbach, the launch director, said, with the countdown clock stopped at nine minutes: "It's not a good day to launch the shuttle so we're going to try again tomorrow".

Sunday's preferred launch time is 3:26 pm (1926 GMT).

This is only the second shuttle mission since the fatal Columbia accident in 2003, and is a critical turning point for Nasa.

Calculated risk

If Discovery is critically damaged on launch or afterward, and astronauts cannot fix the problem in orbit, the seven crew members would stay on the Space Station and wait for rescue, but Nasa chief Mike Griffin has said it could be the end of the shuttle programme.

That would leave the $100 billion International Space Station incomplete and would also mean the United States has no way to carry humans to space.

Columbia broke up in 2003, killing
all its crew members

Griffin decided to launch the shuttle over the objections of the US space agency's head of safety and its top engineer, who wanted the mission delayed to allow more work on the fuel tank and its insulating foam.

The shuttle is the only spacecraft able to lift the heavy components needed to finish the space station.

Nasa has spent $1.3 billion on repairs and safety upgrades to the shuttle fleet since Columbia disintegrated over Texas in February 2003, killing all seven crew.

But the work failed to fully fix a problem with insulating foam on the fuel tank breaking off during launch.

The foam prevents the buildup of potentially damaging ice when the tank is filled with super-cold fuel.

Spacewalks

Astronauts will make two spacewalks during the mission, one to test a new 15-metre extension to the shuttle's robot arm.

During the second spacewalk, astronauts Piers Sellers, a British-born climate scientist, and Michael Fossum, an American making his first spaceflight, will try to fix the space station's broken mobile transporter.

Plans are in place for a third spacewalk if the shuttle has enough fuel to extend the mission for a day.

The space shuttles began flying in 1981 and is scheduled for retirement in 2010 as Nasa develops a new manned spaceship that can travel to the moon and beyond.