The final mission of Nasa's Endeavour shuttle has been delayed by at least 72 hours because of technical problems.
The orbiter is being held on the ground while engineers investigate a hitch in a unit that powers the vehicle's hydraulics.
The youngest of America's reusable spaceplanes is set to deliver a $2bn (£1.2bn) particle physics experiment to the International Space Station (ISS).
US President Barack Obama was due to attend the lift-off in Florida.
Also present at the Kennedy Space Center is Endeavour commander Mark Kelly's wife, Gabrielle Giffords, the Arizona politician shot in the head by a gunman in January.
The lift-off had been targeted for 1547 local time (1947 GMT; 2047 BST), but was postponed just after midday.
Kelly and his crew had already suited up and were making their way to the shuttle. By tradition, their "Astrovan" would normally stop briefly outside Kennedy's launch control centre before moving on down to the pad, but on this occasion the vehicle pulled in front of the building and parked.
The "scrub" was called moments later.
Managers and engineers are now discussing how long a turn-around they need to deal with what appears to be an electrical problem associated with one of the three auxiliary power units in Endeavour.
A heater that prevents freezing in a fuel line leading to the unit stopped working. Another heater was also displaying unusual behaviour.
Nasa managers suspect a short in an electrical switchbox may lie at the heart of the failure but they will not know for sure until engineers can get inside the shuttle to inspect the unit.
"As we say in this business, 'we will not fly before we are ready'," said shuttle launch director Mike Leinbach.
Local police were expecting up to 750,000 people to gather near the Kennedy spaceport to watch the launch. The impeding retirement of the shuttle programme has drawn huge interest in the final flights of the orbiters.
The Discovery ship was the first to bow out with a mission in February. It is now being dismantled and made safe for museum display.
Endeavour is next. After its 14-day trip to the space station, it will be prepared for exhibition at a science centre in California.
The Atlantis orbiter will end the shuttle programme with a mission in June or July.
Nasa hopes by the middle of the decade to be buying crew transportation services from a range of commercial providers.
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