CAPE CANAVERAL, Florida (AFP) – A hydrogen leak in the external fuel tank of the space shuttle Discovery forced NASA to delay its final mission to the International Space Station Friday for the fifth time.

Mission managers said the next launch attempt will not take place until November 30 at the earliest, after technicians found the leak which requires major repairs. NASA had previously set the next launch window at December 1.

"NASA managers have decided to postpone the next launch attempt for space shuttle Discovery to no earlier than November 30 at 4:05 am EST (0905 GMT)," the US space agency said on its website.

The Discovery launch aims to be the fourth and final shuttle flight of the year, and the last scheduled for Discovery, the oldest in the three-shuttle fleet that is being retired next year.

But the mission has been marred by a series of delays brought on by bad weather and equipment problems.

"We know we have a significant leak," said launch director Mike Leinbach, who said NASA specialists were working toward a Monday launch even though he described that as a "challenge."

"Sometime tomorrow we will have a trouble-shooting plan," Leinbach said. "Probably tomorrow afternoon."

Technical teams will not be able to examine the fuel tank up close until Saturday because it must first be emptied of the highly flammable hydrogen, a process which takes about 20 hours.

The leak was found about two hours after operations began to fill the external fuel tanks with liquid hydrogen ahead of the launch, which had been set for 3:04 pm (1904 GMT).

The hydrogen leak was detected at "an attachment point between the external tank and a 17-inch pipe that carries gaseous hydrogen safely away from Discovery to the flare stack, where it is burned off," NASA said.

Heavy rain had scrubbed Thursday's launch of Discovery, after electrical glitches that postponed the launch three times earlier in the week were resolved.

Discovery's 11-day mission with its all-American crew of six will deliver a pressurized logistics module called Leonardo, which will be permanently attached to the space station to provide more storage space.

The shuttle will also bring Robonaut 2, the first human-like robot in space and a permanent addition to the orbiting space station, as well as spare parts.

Two space walks, for maintenance work and component installation, are scheduled.

The Discovery has launched into space 38 times, and NASA aims to retire the shuttle after this, its 39th voyage.

The final official shuttle mission for the entire program is scheduled for Endeavour on February 27, 2011.

However NASA's budget allows for one more shuttle flight which, if the funding can be obtained, may take place in June 2011.

The US space program launched its first shuttle on April 12, 1981.