NASA has cancelled an early Wednesday launch of space shuttle Discovery, blaming an apparently faulty fuel valve.

NASA said Tuesday evening that refuelling had been partly completed when halted but the shuttle astronauts had not boarded.

"Drats!" said astronaut Jose Hernandez in a Twitter update. "24 hour scrub! Won't launch until tomorrow! I guess I get to sleep one more night in a bed!"

On his Twitter page, Swedish astronaut Christer Fuglesang wrote that the earliest possible next launch date is just after midnight on Aug. 28.

If that doesn't work, a second attempt on Aug. 28 could also be made at 11:59 p.m. ET.

The space agency has not rescheduled Discovery's launch.

NASA spokesman Allard Beutel said the valve in question controls the flow of hydrogen from the external fuel tank to the shuttle's main engines.

Discovery had been slated to launch at 1:10 a.m. ET Wednesday from Cape Canaveral, Fla., on a 13-day flight to the International Space Station.

The cancellation was the second of the day. A planned early-morning Tuesday launch was cancelled because of rain and lightning near the launch site.

Last month's Endeavour mission, carrying Canadian astronaut Julie Payette, was postponed several times because of bad weather at the launch site.

Bringing supplies to space station

U.S. Commander Rick Sturckow heads Discovery's crew of seven astronauts, who will deliver almost 8,000 kilograms of equipment and supplies to the International Space Station during the mission.

However, the cancellation cast some doubts whether NASA will be able to meet its stated Aug. 30 deadline to get Discovery into space. If not, the mission could be pushed back to October due to scheduled launches from Japan and Russia to the international space station.

Also scheduled to fly to the station are pilot Kevin Ford, mission specialists Patrick Forrester, Danny Olivas, Jose Hernandez and Nicolle Stott, all from the United States, and mission specialist Crister Fuglesang from Sweden.

Olivas will perform three spacewalks on the mission, one with Stott and two with Fuglesang.

Stott will replace American Tim Kopra and join Canadian Robert Thirsk and the other four members of the space station crew. Kopra is scheduled to return to Earth with the rest of Discovery's crew.

Included in the equipment making the trip into orbit is a treadmill named for comedian Stephen Colbert.

Earlier in the year, Colbert encouraged viewers of his TV program, The Colbert Report, to vote in an online contest to name a future space station room after him. Colbert won the contest, but NASA instead chose to name the piece of exercise equipment after him.

With files from The Associated Press