A faulty fuel sensor aboard the space shuttle Discovery forced NASA on Wednesday to scrub its first attempt to launch a shuttle after the Columbia disaster in 2003.
Mission controllers explained the problem as "a low-level fuel sensor in the external fuel tank, one of a set of four -- two of which are needed to work."
They said it postpones the launch for at least a day.
"It will take some time really to understand what to do to remedy the situation," NASA said.
The current launch window expires July 31; the next begins in September.
Crew members were aboard the orbiter when the announcement was made.
Earlier, it appeared foul weather might postpone the high-profile mission to the international space station.
A new date for the mission was not immediately announced, but the launch window expires July 31; the next begins in September.
The mission will mark the shuttle program's first since Columbia broke apart over Texas during re-entry to Earth in February 2003, killing all seven astronauts aboard.
About 10 miles from the launch site along the Banana River, scores of shuttle watchers -- many wearing shorts and swimwear -- gathered in hopes of a liftoff.
"We'll never experience anything like this back home," Alan Luney said in an accent true to his Northern Ireland home. "We don't have anything at all like this. What we gave to the world was the Titanic, and we all know what happened there."
Many in the crowd sought shelter under umbrellas or had constructed makeshift tents from bedsheets.
Another spectator said she was attracted to the launch by a sense of history.
"I've heard it's going to be one of the last [shuttles] of this model," she said. "I thought it would be a great opportunity to come down here and watch it go off."