STORY: The two-stage Space Launch System (SLS) rocket and its Orion crew capsule were due to blast off from the Kennedy Space Center in Cape Canaveral, Florida, as the first mission of NASA's Artemis program, successor to the Apollo program.
The countdown clock was halted about 40 minutes before the targeted launch time.
The U.S. space agency cited a problem detected on one of the rocket's main engines, during filling of the rocket's core fuel tanks with super-cooled liquid oxygen and hydrogen propellants.
The next possible launch date is Friday, September 2 but that remains to be confirmed.
The maiden voyage when it happens should allow NASA to determine if the craft is reliable enough to carry astronauts.