Nasa has postponed the launch of the space shuttle Atlantis until at least Friday because of a technical problem with its onboard power supply.
Nasa officials hope Atlantis will begin its journey on Friday
Atlantis had been scheduled for a 1229 EDT (1629 GMT) lift-off on Wednesday from Florida's Kennedy Space Center.
Initially Nasa had hoped to reschedule the launch for Thursday.
Officials said the launch had been scrubbed because one of the three fuel cells providing electricity to the shuttle malfunctioned.
The earliest launch window will now be at 1140 EDT (1540GMT) on Friday.
"The launch rules say you need to have three good, operating fuel cells," said Nasa spokesman Bruce Buckingham. "We're going to see if it's something that's a real problem or whether it's something we can rectify."
Two previous launch attempts have been delayed because of a lightning strike and Tropical Storm Ernesto.
The problem was discovered shortly before an overnight meeting to decide whether to start pumping the shuttle's fuel tank with liquid hydrogen and oxygen.
When the fuel cells were powered up, a low voltage reading came from one of the alternating current units. The other two units spiked up to compensate for the lower voltage in the first unit.
The delay puts more pressure on Nasa to get Atlantis off the ground this week or postpone the launch until late October in order not to interfere with a Russian mission to the space station later in the month.
Atlantis is on a mission to resume construction of the International Space Station, which was halted following the Columbia disaster in 2003.
The shuttle's six-strong crew will deliver and fit the P3/P4 truss, a 17-tonne segment of the space station's "backbone" that includes a huge set of solar arrays and a giant rotary joint to allow them to track the Sun.
ISS: ORBITING OUTPOST
Construction work has been on hold for four years
16 nations contribute to the ISS, including the US, Russia, Japan, Canada, Brazil and European Space Agency states
The ISS will eventually be the size of a football field
The arrays will be the second of four sets, and will span 73m (240ft) when fully extended.
They will provide power for three science laboratories, two living chambers and other systems onboard the ISS. They effectively double the station's current ability to generate power from sunlight.
The half-built $100bn (£52bn) space station must be completed before 2010, when the shuttle fleet is due to be retired.
A failure to launch this week could delay the Atlantis mission until late October.