The launch had been scheduled for 7:17 am (1117 GMT) from Cape Canaveral in Florida. Now the earliest opportunity for the shuttle's liftoff will be on June 17, officials said.

The crew's commander, Mark Polansky, wrote on the micro-blogging website Twitter that his crew would await further word from NASA chiefs on how to proceed.

"What happens now? We work 2 a Wed launch while senior management meets to decide on the plan. My crew stays in quarantine at KSC (Kennedy Space Center) for now."

The leak in a venting system was detected near the Ground Umbilical Carrier Plate, which is attached to the external tank at its intertank area, the National Aeronautics and Space Agency said in a statement.

The system is used to carry excess hydrogen safely away from the launch pad at NASA's Kennedy Space Center in Florida.

"This is NOT a shuttle problem, but a pad one," Polansky stressed.

Shuttle managers have set a meeting for 2:00 pm Sunday (1800 GMT) to discuss the repair options and Endeavour's launch attempt opportunities.

It will now take the space agency 24 hours just to empty Endeavour's external tank, which contains some two million liters of liquid hydrogen and oxygen.

But officials said the new target date of June 17 may also present a conflict as NASA has scheduled for that day the launch of the Lunar crater observation and sensing satellite, which is due to lift off on top of an Atlas V rocket from Cape Canaveral Air Force Station, Florida.

If a launch then proves unattainable, NASA has said the only next available blast-off date would July 11 at the earliest.

The leak was similar to one that occurred during the first launch attempt of the space shuttle Discovery in March.

But mission manager Mike Moses acknowledged that even now NASA did not know what had caused that leak.

"We did not really find anything, but obviously something is going on, the second time over three flights," he said.

When Endeavour eventually arrives the International Space Station is set to be a temporary home to 13 astronauts -- the first time so many people have stayed on the 100-billion-dollar orbiter at once.

The six US astronauts and a Canadian female astronaut that Endeavour is expected to eventually bring to the ISS will join another US astronaut and one more from Canada, as well as two Russians, a Belgian and Japan's Koichi Wakata who are currently living on the ISS.

Mission specialist Tim Kopra is set to replace Wakata, who will return to Earth after a three-month stint at the orbiting outpost.

Construction began on the ISS a decade ago, and the push is on to complete building before NASA ends its shuttle missions in September 2010.

Endeavour's crew are tasked with installing the final elements of the Japanese laboratory Kibo during their 16-day mission.

The station has become a sophisticated platform for scientific experiments after the installation of a European laboratory last year and the arrival of the high-tech Japanese lab currently being completed.

When Endeavour finally lifts off, it will be the 32nd mission to the ISS, which orbits 350 kilometers (220 miles) above Earth, and the last of three missions to assemble the Kibo laboratory.

Over the five planned spacewalks lasting some 32.5 hours, the astronauts will install a permanent 1.9-tonne platform to Kibo, which will serve as one of the station's porches for conducting experiments in space.

NASA is seeking to make maximum use of its flights to the space station with only another seven planned after the Endeavour's trip and before the shuttles are retired.

Senior NASA official Bill Gerstenmaier has offered assurances that the space station can host up to 13 astronauts at once, after a fourth solar panel was erected along with other equipment including toilets, a kitchen and a machine to recycle urine into drinking water.

But he acknowledged: "It's a very complex and challenging mission for the team."

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