Washington - The launch of the space shuttle Discovery planned for later Wednesday was abruptly cancelled just hours ahead of blast off.

NASA called off the launch after it discovered a leak in a hydrogen vent line between the shuttle and its external fuel tank. The US space agency made the decision ahead of a planned 9:20 pm (0120 GMT Thursday) launch.

Technicians had been filling the tank with fuel and would now have to completely drain it. A new launch time was set for 8:54 pm Thursday (0054 GMT Friday).

The launch had already been delayed for weeks to allow NASA time to examine valves that control the flow of gaseous hydrogen from the spacecraft's engine to its external fuel tank. A valve was found to be damaged after the space shuttle Endeavour's November flight, prompting NASA to delay Discovery's mission as a precautionary measure.

The Discovery is to deliver the last solar panel to the International Space Station on a construction mission that will enable it to double the size of its crew to six astronauts.