"We gave it our best shot and didn't get clear and convincing evidence that the cloud ceiling had cleared for us," launch director Mike Leinbach told the crew. "We have to declare a scrub at this time." The shuttle commander, Mark Polansky, responded. "Thanks to the team for all the hard work, try not to be too disappointed and we will be ready to support for the next time we get a chance," he said.

The shuttle's external fuel tank was filled with two million liters (500,000 gallons) of liquid hydrogen and oxygen when the launch was scuttled, after a tense half-hour of on-again-off-again weather conditions.

"We had to scrub the launch tonight because the cloud ceiling exceeded our requirements," said NASA spokesman Bruce Buckingham.

NASA has described the shuttle mission as the most complex to date, with three space walks scheduled to rewire the ISS and install a new 11-million-dollar truss segment.

The Discovery mission was to be the first night shuttle launch since the Endeavour flight in November 2002.

The three shuttle launches following the February 2003 Columbia tragedy -- in August 2005, and in July and September 2006 -- were scheduled during daylight so that ground and shuttle cameras could closely photograph the spacecraft's exterior tank in case pieces of thermal insulation or ice broke off during liftoff.

A piece of insulating foam damaged Columbia's heat-shield shortly after launch which led to the shuttle disintegrating upon re-entry to Earth's atmosphere, killing all seven astronauts aboard and grounding the shuttle program for more than two years.

Discovery's mission will be the third and final shuttle launch of the year, and the fourth since the Columbia disaster.

NASA has planned 14 shuttle missions -- including this one -- through 2010 to finish building the International Space Station.

The weather forecast for a Saturday launch, however, was discouraging, with only a 30 percent chance of favorable conditions.

The space agency has set a window for rescheduling the liftoff that lasts until December 26, but after December 17 it would have to reprogram the shuttle's computer to adjust to the date change at the end of the year.

After December 26, the Discovery mission would have to wait until mid-January for another chance at liftoff.

One of the astronauts aboard Discovery, American Sunita Williams, will be staying at the ISS. She will repace German Thomas Reiter of the European Space Agency, who arrived at the Space Station in July and will be returning to earth at the end of Discovery's mission.