Controllers at the the Kennedy Space Centre in Cape Canaveral, Florida, halted the fuelling operation for Artemis 1 on Monday morning.
NASA said its engineers were rectifying the issue – but similar leaks hindered countdown tests in April and June.
Managers said they would not know for sure whether the leaks had been resolved until attempting to load the rocket’s tanks with nearly one million gallons of super-cold fuel later on Monday.
A statement reads: “Teams continue to troubleshoot a liquid hydrogen leak at the mating interface with the core stage.
“After manually chilling down the liquid hydrogen as part of troubleshooting efforts, they are in fast fill operations.”
Artemis 1 at the Kennedy Space Centre in Cape Canaveral, Florida
JOE SKIPPER
NASA's Artemis 1
STEVE NESIUS
The uncrewed 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.
The Artemis 1 mission will see the first launch of the new 322ft (98m) tall Space Launch System (SLS) rocket, which the agency says is the world’s most powerful rocket to date.
It will take the Orion capsule, powered by the Airbus-built European Service Module (ESM), into the moon’s orbit.
Airbus engineer Sian Cleaver is industrial manager for the ESM, and as a child dreamt about being involved in human spaceflight before getting a master’s degree in physics and astronomy from Durham University.
She said: “I’m ridiculously excited, and I think everybody on the team is.
“There’s years and years of a labour of love into this project.
“This is the first time that we will have seen one of our European service modules flying in space and going to the moon.
“I think a lot of us couldn’t quite believe it – we’ve now got the go for launch.
“Now, I think it’s really sinking in that this is reality, this is happening, and it’s going to really start this whole new chapter of space exploration, and going to the moon.
“We’re on the brink of something really exciting now.”
Ms Cleaver said the last time humans went to the moon, some 50 years ago, it was about proving that it could be done.
But she says the new mission is about proving people can go there for longer and more sustainably.
It will also assess whether some infrastructure can be built on and around the moon, allowing humans to survive on another planetary body.
The mission duration is 42 days, 3 hours, 20 minutes, and in total the capsule will travel 1.3 million miles, before splashing down on October 10.
The UK is part of the Artemis programme, making contributions to the Lunar Gateway – a space station currently in development with the European Space Agency – working alongside the US, Europe, Canada and Japan.
The Artemis mission will be tracked in the UK from Goonhilly Earth Station in Cornwall.
Libby Jackson, exploration science manager at the UK Space Agency, said: “The first launch of the Artemis 1 SLS rocket is an important step for the global space community as we prepare to return humans to the moon.
“The Artemis programme marks the next chapter of human space exploration and we look forward to continued involvement as it comes to life.”