CAPE CANAVERAL, Florida (Reuters) - Space shuttle Discovery was being fueled for liftoff on Thursday on its final mission to carry supplies and spare parts to the International Space Station.

Workers at the Kennedy Space Center in Florida began pumping supercold liquid oxygen and liquid hydrogen into Discovery's fuel tank in preparation for a launch attempt at 4:50 p.m. (2150 GMT)

The preparations came after months of delays for the last flight of the Discovery, the first of three shuttles NASA plans to retire this year as it phases out its shuttle program.

NASA attempted to launch Discovery on November 5, but was stymied by a leak in a hydrogen vent line. Later, an unrelated and far more serious problem surfaced with support beams inside the fuel tank.

Engineers eventually determined a combination of problems, including flawed materials, caused some of the beams to crack and devised a sort of metal bandage to reinforce the trouble spots.

"It's going to perform perfectly fine," said launch director Mike Leinbach.

The shuttle carries a storage room for the space station, a $100 billion project of 16 nations nearing completion after more than a decade of construction 220 miles above the Earth.

Discovery also is transporting a platform to house spare parts outside the station and equipment and supplies. The cargo includes a humanoid robot, known as Robonaut 2, that will be set up in the U.S. laboratory for a trial run.

The flight will be the 39th and final space mission for NASA's oldest surviving orbiter -- shuttles Columbia and Challenger were lost in accidents that claimed the lives of 14 astronauts.

The remaining three ships are being retired due to high operational costs and to free up money to develop new vehicles capable of traveling beyond the space station's orbit.

NASA plans to launch shuttle Endeavour on its final flight in April and end the program with a final cargo haul to the station over the summer aboard Atlantis, though funding for that mission has not yet been allotted.

"Landing day is going to be tough," Leinbach said. "Landing day of Discovery, and then Endeavour and especially Atlantis, the last mission, you'll see a lot of people on the runway who will probably choke up some because it's the end of a 30-year program that not only have we worked in and made our livelihoods in, but we've grown to love and appreciate and feel like we're doing something special for the country and, really, the world."

The delay launching Discovery cost the crew its lead spacewalker, Tim Kopra, who was injured in a bicycle accident last month. He was replaced by Stephen Bowen.

The five-man, one-woman crew includes commander Steven Lindsey, pilot Eric Boe, spacewalkers Bowen and Alvin Drew, along with Michael Barratt and Nicole Stott. They are scheduled to spend 11 days in orbit.

(Editing by Kevin Gray and Eric Beech)