A fuel gauge that mistakenly read full instead of empty forced National Aeronautics and Space Administration (NASA) to call off Wednesday's launch of the first shuttle flight in two-and-a half years, a bitter disappointment to the space agency in its bid to recover from the Columbia tragedy.

The launch was delayed until at least Saturday, and the postponement could last much longer depending on the repairs needed.

Shuttle managers said it was unclear whether Discovery could be fixed at the pad or would need to be returned to the hangar for more extensive repairs. They expected to have a better idea Thursday.

A similar problem cropped up intermittently during a fuelling test of Discovery back in April. The external fuel tank, along with cables and other electronics, were replaced, and even though NASA could not explain the failure, it thought the problem was resolved and pressed ahead with launch.

At a news conference of grim-faced NASA officials, deputy shuttle programme manager Wayne Hale defended that decision.

"We felt like we had a good system," he said.

"We became comfortable as a group, as a management team, that this was an acceptable posture to go fly in," he added, "and we also knew that if something were to happen during a launch countdown, we would do this test and we would find it. And guess what? We did the test, we found something and we stopped. We took no risk. We are not flying with this."

The seven astronauts had barely climbed aboard Discovery for their journey to the international space station when NASA halted the countdown with less than two-and-a-half hours to go. Until then, the only threat to NASA's first mission since the 2003 Columbia disaster was bad weather.

"Appreciate all we've been through together, but this one is not going to result in a launch attempt today," launch director Mike Leinbach informed his team.

NASA has until the end of July to launch Discovery; otherwise it must wait until September because of the position of the space station and NASA's desire to hold a daylight lift-off in order to photograph the spacecraft during its climb to orbit.

When the shuttle finally takes off, the astronauts will test new techniques for inspecting and repairing cracks and holes similar to the damage that doomed Columbia.

Thousands of people had descended on the space centre for the launch, including John Glenn, the first American to orbit the Earth, members of Congress, and family members of the seven fallen Columbia astronauts. Lawmakers and others refrained from second-guessing NASA's decision to press ahead before it had gotten to the bottom of the fuel gauge problem.

"I'm disappointed for all of us," said Senator Bill Nelson, a Florida Democrat, who as a congressman was on the shuttle right before the 1986 Challenger launch explosion. But he added, "The system is working like it should."

Just a day earlier, NASA was embarrassed when a lightweight plastic window cover fell off the shuttle on the launch pad and caused damage to some of its thermal tiles - the very thing that NASA had worked so hard to avoid after Columbia's wing was pierced at lift-off by a chunk of foam insulation from the fuel tank. Discovery's tiles were quickly replaced.

Since the Columbia tragedy, NASA has worked to fix its "safety culture." The space agency said it has had frank and vigorous discussions about the upcoming flight - including the fuel gauge problem - and encouraged engineers to speak up.

The faulty gauge reading cropped up after the tank was filled with more than 500,000 gallons of liquid hydrogen and oxygen. Launch controllers ran a test to check out all four hydrogen-fuel gauges in the tank; when they sent a command mimicking an empty tank, three indicated empty, while one stayed stuck on full.

It was a clear violation of the launch rules, Hale said, and it took just five minutes of discussion for managers to agree on a postponement.

Source: China Daily