Hit by more valve trouble, NASA postponed the launch of space shuttle Discovery just hours before it was to head to the international space station Wednesday because of a hydrogen gas leak.

The potentially catastrophic leak was in a different part of the system that already had caused a vexing one-month delay.

Shuttle managers put off the launch until at least Sunday and indicated that Monday might be more likely.

The latest delay means Discovery's two-week flight must be shortened and some spacewalks cut out of the mission. That's because Discovery needs to be gone from the space station before a Russian Soyuz rocket blasts off March 26 with a fresh station crew.

If Discovery isn't flying by Tuesday, it will have to wait until April. That almost certainly would bump the succeeding space shuttle missions as well as plans to double the size of the space station crew at the end of May, said Mike Moses, chairman of the mission management team.

Mission Control radioed the news to the three space station residents Wednesday evening. Commander Mike Fincke took it in stride, saying he'd rather see the shuttle this month than next.

"But more importantly, we'd rather see it up safely, so we understand," Fincke said.

NASA does not want a shuttle at the space station at the same time as a newly arrived Soyuz because of the tremendous workload it would put on all the crews.

The gaseous hydrogen began leaking just as the launch team was close to wrapping up the loading of Discovery's external fuel tank for a late-night liftoff. The seven astronauts had yet to board the spaceship.

Launch director Mike Leinbach said the problem appears to be with a valve in the fuel tank for venting hydrogen gas overboard. Whenever the valve was opened to relieve pressure in the tank, hydrogen gas leaked out into the air.

In the past, the problem would go away with a simple closing and reopening of the valve.

"This time it didn't," Leinbach said. He noted that this was the largest such leak ever detected.

Engineers won't know exactly what's wrong until they are able to reach the area Thursday afternoon. Leinbach said he's "99.9 percent sure" that the problem is not in the tank itself but in the outside connections, which makes for an easier, quicker repair.

Officials acknowledged it was a huge disappointment, especially coming on the heels of all the earlier delays.

"Let's be honest. We'd rather be launching than scrubbing," Leinbach told reporters. "But our business requires perfection, and our vehicle is not perfect today and so we're going to stand down. We're going to fix the vehicle and fly when it is perfect."

The last thing NASA wants around a space shuttle is leaking hydrogen, which is highly flammable and could lead to a catastrophic explosion.

Discovery's liftoff originally was targeted for Feb. 12, but concern about its three hydrogen gas valves resulted in four delays.

Shuttle managers said they're convinced after extensive testing that the valves in Discovery's engine compartment are safe and won't break like one did during the last shuttle launch in November. These valves — part of the main propulsion system — control the flow of hydrogen gas into the fuel tank in order to maintain proper tank pressure.

Tucked aboard Discovery is 31,000 pounds of framework that holds two folded-up solar wings and a radiator. The space station already has six electricity-producing wings; the two going up will be the last ones and elevate the orbiting outpost to full power.

Four spacewalks had been planned for the shuttle mission. Moses said much of that work could be handed off to the space station crew, after the shuttle leaves, in order to get the solar wings up this month and avoid the logjam that would be created if Discovery has to wait until April.

NASA will consider the mission a success as long as the astronauts can deliver and install the solar wings, drop off a new urine processor for the space station's water-recycling system, and carry out a space station crew member swap.