NASA announced that today's attempt to launch the space shuttle Atlantis has been officially scrubbed.

The launch target was scheduled for today at 3:21 p.m. EST from the Kennedy Space Center, Fla. NASA managers decided to attempt the launch after a meeting in which they reviewed the data on a problem with a fuel cutoff sensor system inside the shuttle and its external fuel tank.

"Liquid hydrogen sensor number three has failed. This is not good news," launch commentator George Diller said just after fueling began.

"Confirmation now that we have scrubbed for the day," Diller added about a half-hour after the sensor problem became apparent. "That direction coming from LeRoy Cain, chairman of the Mission Management Team, and Doug Lyons, the STS-122 launch director."

On Sunday morning NASA began refuelling the Atlantis shuttle, Of the four engine cutoff sensors, ECO sensor number three gave false readings. NASA's current Launch Commit Criteria require that all four sensors function properly.

"In a way this could be a good thing," said NASA spokesman George Diller, adding that it "may very well help us get to the bottom of this problem."

Atlantis’ scheduled launch on Thursday was delayed after two ECO sensors gave false readings. A third sensor failed after the tank was drained of fuel. Known as Engine Cut-Off (ECO) sensors, the instruments sit on the bottom of Atlantis' 15-story external tank and serve as liquid hydrogen fuel gauges that ensure a shuttle's three main engines shut down before their hydrogen supply runs dry after liftoff. Engineers considered flying even if the sensors failed again but managers decided it would be too risky

Fuel sensor problems have caused considerable launch delays since the retooling of troublesome tanks after the 2003 Columbia disaster.

Last year NASA loosened its launch rules to require only three of the four fuel gauges to be working before liftoff. Managers went back to the four-of-four rule Saturday after concluding that Atlantis' system was suspect and that it would be too risky to attempt a launch without every single gauge functioning.

If the shuttle is not flying by week's end, the mission will be delayed until January because of unfavorable sun angles and computer concerns. NASA did not say anything about another launch attempt.

Atlantis should carry the European-developed Columbus laboratory and attach it to the International Space Station. Seven astronauts, including two from the European Space Agency, should fly aboard Atlantis.

Columbus is about 23 feet long and 15 feet wide, allowing it to hold 10 "racks" of experiments, each approximately the size of a phone booth. Five NASA racks will be added to the laboratory once it is in orbit. Each rack provides independent controls for power and cooling, as well as communication links to earthbound controllers and researchers. These links will allow scientists all over Europe to participate in their own experiments in space from several user centers and, in some cases, even from their own work locations.

For NASA every mission is critical because the US space agency is rushing double capacity on the space station by 2010, when the ageing shuttle fleet is set to retire.

Unfortunately, NASA already is behind schedule on design and development of the International Space Station, though NASA Administrator Michael Griffin said the agency has executed the plan well overall.

"We the United States, as the senior partner in the space station coalition, did not plan it well," Griffin said on the eve of Columbus' launch. "It has taken far too long and I'll just leave it at that."