NASA postponed the launch for at least four days.
Monday had meant to be the start of a planned six-week uncrewed voyage around the moon and back, 50 years after Apollo's last lunar mission.
The countdown clock was halted about 40 minutes before the targeted launch time, as the rocket and its Orion crew capsule awaited liftoff from the Kennedy Space Centre in Cape Canaveral, Florida.
The US space agency detected a problem on one of the rocket's main engines, after launch teams had begun filling its core fuel tanks with super-cooled liquid oxygen and hydrogen propellants.
NASA did not give a new launch date but said its first available backup launch opportunity was set for Friday September 2.
A NASA statement reads: "The launch of Artemis I is no longer happening today as teams work through an issue with an engine bleed."
The unmanned flight marks the next chapter in putting humans back on the moon, and is the first in NASA's Artemis programme.
There will be astronauts on board for subsequent missions, with the first crewed flight into space scheduled for 2024.
NASA expects the first Artemis astronauts to land on the moon in 2025.
NASA's next generation moon rocket
JOE SKIPPER
NASA's Orion crew module for the Artemis 3 mission
STEVE NESIUS
