The launch was scrubbed for a fourth time in the early hours after NASA engineers discovered that one of the sensors on the spacecraft's external hydrogen fuel tank had failed.

"The mission management team has decided to postpone the launch until January 2 at the earliest," NASA spokesman Allard Beutel said.

The Atlantis crew of seven was preparing for an 11-day mission to fly Europe's Columbus laboratory to the space station, orbiting hundreds of miles above Earth.

They had not yet entered the shuttle before the launch was called off, and will now return to NASA headquarters in Houston later Sunday, the agency said.

"We were ready to fly, but understand that these types of technical challenges are part of the space program," a crew statement said.

"We hope everyone gets some well-deserved rest, and we will be back to try again when the vehicle is ready to fly."

Atlantis's launch had already been postponed from Thursday, after two of the sensors gave false readings during fueling. Under NASA rules, all four sensors have to be working properly for the launch to proceed.

If the sensors on the external tank register low fuel, they shut down the main engines on the shuttle itself, and NASA bosses said technicians need more time to find out what was causing the faulty readings.

The crew included two European Space Agency astronauts -- Hans Schlegel of Germany and Frenchman Leopold Eyharts. Eyharts was scheduled to stay on the ISS for two and a half months to prepare Columbus for future scientific work.

Until now, only the United States and Russia have had their own laboratories at the ISS.

With Columbus, Europe hopes to become an integral part of the only functioning orbital outpost, whose experiments with gravity are considered essential to prepare humans for long-term life and work in space, and journeys to Mars and beyond.

The Japanese laboratory Kibo, the fourth planned component of the ISS which is to be the largest and most sophisticated of all, is meant to be delivered in stages over subsequent shuttle flights.

The first of the Kibo flights is scheduled for around February 14, on the Endeavour shuttle, and a second is due in April on the Discovery craft. Other shuttle flights are scheduled for August and September.

Bill Gerstenmeyer, NASA's associate director for space operations, played down the impact of the lengthy Atlantis delay on next year's shuttle launches, some of the last before the fleet's scheduled retirement in 2010.

"We are all a little bit disappointed we did not to get to see a launch today," Gerstenmeyer told a news conference.

But he added: "If you look at what this means moving into January from the big-picture standpoint, it's not that big an impact to us overall, it won't impact the next mission, the February flight."

Initially it was planned that Columbus would be flown to the ISS at the end of 2004.

But the tragic end of the shuttle Columbia in February 2003 grounded the three remaining shuttle orbiters for two years, which in turn delayed the laboratory's launch.

Columbus will be controlled from the German Aerospace Center in Oberpfaffenhofen, close to Munich. In all, 10 European countries are participating in the program with Germany by far the biggest contributor.

The laboratory will allow astronauts to conduct hundreds of experiments a year, notably in areas of biotechnology, medicine, materials and fluids.

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