A leak of hydrogen fuel at the launch pad forced NASA to call off Thursday's planned liftoff of shuttle Atlantis on a space station construction mission.

Officials said the leak occurred in a vent line outside the shuttle, at the base of the launch platform. The super-cold, highly flammable fuel could be seen in a NASA videotape, streaming out in large, white clouds of gas and dissipating into the air.

The fuel vapors escaped from a pipe that appeared to have broken or come loose at a fitting.

No one was right at the pad because the fueling operation is hazardous, but an engineer spotted the leak on a video image, launch director Mike Leinbach said. NASA immediately halted the fueling, about an hour after it had begun. Atlantis' huge external fuel tank was less than 20 percent filled at the time.

Because the leak will take time to repair, the earliest that a new launch attempt could be made would likely be Sunday, Leinbach said. He said he didn't think the leak had created much of a danger.

The leak was not in the plumbing that feeds liquid hydrogen into the external tank, but rather in a line that vents the fuel vapors out of the tank and thereby prevents a buildup of excess pressure. The fact that the leak occurred outside the shuttle makes the repairs much easier -- and quicker.

Fuel leaks grounded the shuttle fleet for months in 1990, but those leaks happened inside the spaceships.

The seven astronauts were hours away from boarding Atlantis for the late-afternoon launch and had not even begun to suit up. The mission is to last 11 days and include four spacewalks.

Atlantis had been scheduled to lift off at 5:13 p.m. with the newest piece of the international space station, an elaborate 44-foot girder.

The launch time was not revealed by NASA until late Wednesday afternoon. Under a security policy adopted last month, the space agency will no longer announce shuttle launch times until 24 hours in advance, in hopes of thwarting terrorists.