A malfunctioning fuel tank sensor is said to have caused the delay, and shuttle program manager Wayne Hale said replacing the sensor meant that NASA would have to put off the launch originally scheduled for May 10.

The sensors are designed to ensure that the shuttle's main engines shut down before they drain liquid oxygen and liquid hydrogen propellants from the fuel tank. Malfunctions with the sensors could cause the main engines to stop working before the shuttle is in orbit.

They also risk triggering an explosion if the sensors fail to cut off the engines when the shuttle actually runs out of fuel due to a leak or other problems.

The next opportunity to launch Discovery to the International Space Station will be between July 1 and 19.

The planned launch in May is the second shuttle flight since the Columbia disaster in February 2003, which was blamed on insulation foam falling off the fuel tank during the re-entry into Earth's atmosphere.

NASA resumed shuttle flight in July last year, but the foam problem emerged again and forced the U.S. space agency to ground the shuttle fleet till now. Enditem