Deputy space shuttle programme manager LeRoy Cain: "We're going to step back and figure out what the problem is"
A new leak during fuelling has forced Nasa to postpone the launch of space shuttle Endeavour until July at the earliest.
The shuttle was due to blast off from Florida early on Wednesday, but hydrogen gas leaked from a vent line on its external fuel tank, officials said.
A previous gas leak postponed the shuttle's launch on Saturday.
Endeavour had been due to deliver part of a Japanese laboratory to the International Space Station (ISS).
"At 1.55am (05.55 GMT) launch managers called a scrub, cancelling today's planned launch of space shuttle Endeavour on its STS-127 mission," a Nasa statement said.
"Despite troubleshooting efforts, engineers were unable to achieve a decrease in the liquid hydrogen leak."
For an hour after the leak appeared during fuelling, engineers tried to fix the problem through remote commands. But they were finally forced to call off the pre-dawn launch from Florida's Kennedy Space Center.
"I sure wish we could have rewarded them and the astronauts and everybody else with a launch this morning," said assistant launch director Mike Leinbach.
"But the leak was way out of spec again, and so we were just not comfortable pressing on."
The shuttle's commander Mark Polansky commented: "It's a reminder that spaceflight is not routine."
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