Nasa called off the launch of the space shuttle Endeavour with just hours to spare on Saturday morning amid alarm over a leaking fuel line.

The space agency halted the countdown clock less than 7hours before the scheduled 7.17am lift-off from Kennedy Space Centre at Cape Canaveral, Florida, after highly volatile hydrogen gas was found to be escaping from the external tank during the final stages of the fuelling process.

The delay is critical for Nasa, which had been set to launch Endeavour on a 6.6 million-mile mission to continue construction of the International Space Station.

It is now unlikely that the shuttle will lift off before next Saturday as repairs are expected to take several days, followed by a stand-down period to make way for the scheduled launch on Wednesday of two unmanned probes on a $600 million mission to the Moon.

The shuttle's 7 crew members, led by Commander Mark Polansky, 53, had been preparing to don their pressurised spacesuits and head out to the launch pad when word came from the launch control room that all was not well.

"Got the word that we're not going today," Cdr Polansky noted in a brief note posted to his Twitter site just before 1am local time.

"That's part of business. Most important thing is to not launch til everything is ready," he added.

The problem is a repeat of the same issue that delayed the launch of Endeavour's sister-ship Discovery in March. The hitch, which showed up in the final stages of fuelling the external tank with 500,000 gallons of liquid hydrogen and liquid oxygen, adds further pressure to Nasa's cramped timetable for completing 8 more shuttle missions before retiring the fleet in 2010.

"Everything was going perfectly fine," said launch director Mike Leinbach at a 2am press conference at Cape Canaveral, adding: "There's no way we could have continued. We did out standard troubleshooting to try and clear it up but that didn't work."

Hydrogen, he said, is "a commodity that we just don't mess with. we just were not comfortable at all with pressing on."

Endeavour's mission, once it gets off the ground, will set a record for the biggest get-together in space. On arrival at the $100 billion International Space Station, the crew will be welcomed aboard by the 6-strong team already living there, creating the biggest gathering ever held in space.

They will work together on a gruelling series of tasks that will include installing the final segment of the station's Japanese laboratory module – known as Kibo – to create an external platform on which scientific experiments can be conducted in the vacuum of space.