(Adds shuttle delayed until Saturday)

By Maggie Fox

CAPE CANAVERAL, Fla., Dec 6 (Reuters) - NASA officials worried about tricky fuel sensor readings postponed a planned launch of the space shuttle Atlantis on Thursday, saying it could not go before Saturday at the soonest.

They said they believed a wiring problem caused two of four fuel tank sensors to fail and were trying to determine if it would be possible to either fix the problem or launch the mission safely anyway.

"We no longer have an opportunity to launch tomorrow. Our earliest opportunity that we are working to is Saturday," said LeRoy Cain, head of the shuttle Mission Management Team.

NASA has until Dec. 13 to launch the 11-day mission this year and take the European Space Agency's Columbus science laboratory to the International Space Station. Atlantis must have time to leave the station before the angle of the sun's rays change enough to overheat the orbiter.

The weather was fine and everything had seemed ready for the third on-time launch this year. But on Thursday morning two of the four engine cutoff, or ECO, sensors inside the liquid hydrogen section of the tank failed a routine prelaunch check.

"We have these sensors at the bottom of the tank, kind of like the fuel tank in your vehicle, and they tell us when we are about to run out of fuel," Cain told reporters.

The shuttle uses more than 500,000 gallons (2.2 million litres) of frozen hydrogen and oxygen to get into orbit. If the engines keep firing after the tank is empty, it could spark an explosion.

Engineers and other experts have been meeting to discuss the problem and were ordered to go home and sleep on it.

"I can almost guarantee you we will have some new thinking, too, after we let this team go home and rest," Cain said.

The first mission after the 2003 loss of the shuttle Columbia and its seven astronauts, in July 2005, was delayed 13 days due to sensor glitches. Cain said the sensor system has parts added over the years to boost safety.

ESA has waited more than five years for this launch. Its initial launch was postponed when NASA grounded the shuttle fleet for safety upgrades after the Columbia disaster.

Germany's Hans Schlegel and Leopold Eyharts of France are scheduled to join five U.S. astronauts on this mission -- commander Stephen Frick, pilot Alan Poindexter, flight engineer Rex Walheim, Leland Melvin and Stanley Love.

They will help install Columbus, a 27-foot-(8.2-metre)long, 15-foot-(4.6-metre)diameter laboratory set up for experiments in biology, physics and medicine. (Reporting by Maggie Fox and Irene Klotz)

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