Nasa called off the launch of space shuttle Discovery because of a bad fuel valve.
Launch officials halted the countdown late this afternoon, midway through the fuelling process.
The seven astronauts had not yet boarded the shuttle for tomorrow morning's scheduled flight to the international space station. A new launch date was not immediately set.
Nasa spokesman Allard Beutel said engineers were trying to open and shut the fill-and-drain valve in Discovery's engine compartment when it appeared to be broken. The exact condition of the valve is not yet known.
The valve is used for the flow of liquid hydrogen from the external fuel tank to the main engines.
It is the second launch delay in as many days for Discovery. This morning's planned launch was called off because of thunderstorms.