NASA says the final launch of Discovery remains on track for Feb. 3, despite finding more cracks in the fuel tank's support structures. (Tony Gray/NASA)

NASA has ordered additional repairs to the fuel tank of the shuttle Discovery after finding four more cracks, the space agency has announced.

The development raises the possiblity that the mission launch — already delayed from Nov. 1 — could be pushed back further.

Following a pre-Christmas roll-back of the shuttle to allow for a closer examination of the tank, engineers used X-rays to discover four small cracks in three stringers on the external fuel tank opposite from the Discovery.

"Managers elected to repair those cracks in a similar fashion to repairs made on cracks discovered after the Nov. 5 launch attempt," a NASA statement said. "Any further work will be evaluated thoroughly early next week after additional data is reviewed."

NASA said the repairs are likely to take two or three days. but so far says Discovery is still on track for a Feb. 3 launch.

The agency says that date could be pushed back if additional repairs are needed.

Discovery was grounded in early November after a hydrogen gas leak was initially discovered during fuelling. Cracks were later found in the shuttle's fuel tank beneath a crack in the tank's insulation layer.

Discovery's launch has been repeatedly delayed since the fall — once by weather and several times for technical reasons.

The 39th and final flight of Discovery will carry seven astronauts and supplies to the International Space Station.

NASA's shuttle program will come to an end later in 2011 — after two and possibly three more missions. After that, Russia will carry astronauts to the space station.

NASA hopes that private industry will be ready by 2015 to step in to ferry astronauts and cargo to the space station.