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.

NASA technicians check the
mobil launch platform (MLP)
for the launch of Space shuttle
Endeavour on mission
STS-127 at Kennedy Space
Center Photo: EPA

The space agency halted the countdown clock less than seven hours 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 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 eight 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 six-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.