Washington  - A potentially disastrous fuel leak prompted NASA to issue a second delay notice Wednesday for space shuttle Discovery, pushing back the earliest launch time to Sunday.

Just hours before a planned Wednesday night (early Thursday GMT) launch, NASA delayed launch time to 0054 GMT Friday due to the leak.

The new launch target time is 2343 GMT Sunday, NASA said on its website. But the decision to launch "is dependent on the work necessary to repair the problem," NASA said.

The ageing Discovery, which is more than 20 years old, was to bring the first long-term Japanese crew member, Koichi Wakata, to the International Space Station.

NASA cited a leak in the hydrogen vent line between the shuttle and its external fuel tank. The system is used to carry excess hydrogen safely away from the launch pad.