The mission management team was scheduled to meet early in the day to give a "go" to fill Endeavour's massive external fuel tank.

If managers give their approval, fueling would begin at 8:30 am (1230 GMT), before the shuttle's scheduled 6:03 pm (2203 GMT) lift-off, carrying seven crew members to the orbiting International Space Station (ISS).

The shuttle launch has been cancelled three times since Saturday due to inclement weather and two earlier attempts were aborted after potentially hazardous fuel leaks were discovered, apparently caused by a misaligned plate linking a hydrogen gas vent line with the external fuel tank.

Lightning storms and fuel tank problems left the cash-strapped US space agency footing 4.5 million dollars in extra costs attached to the scrapped launch attempts, as officials kept their fingers crossed that they will finally have a success.

"The cost of a scrub is approximately one million dollars," said spokesman Allard Beutel at NASA's Kennedy Space Center at Cape Canaveral, Florida.

Along with the cost of filling, draining and then refilling the external tank so many times with specialized liquid hydrogen and oxygen fuel, expenses also skyrocketed due to overtime pay for NASA employees and other workers at the space centers here.

But Beutel said the added costs were "marginal" in NASA's overall operating budget. The agency says Endeavour alone, built to replace the shuttle Challenger, cost some 1.7 billion dollars.

Endeavour was originally scheduled to launch June 13 but a liquid hydrogen leak twice postponed it last month.

In addition to the possible Wednesday blast-off, a launch was also being considered for Thursday, the last possible date before interfering with the July 24 lift-off of the Russian cargo craft Progress to the ISS, launch integration manager Mike Moses said.

A Thursday launch date would force NASA to abandon one of five spacewalks planned for Endeavour's mission.

If the shuttle does not take off on Wednesday or Thursday, the next launch window would begin on July 26.

NASA conducted repair work that they hoped would pave the way for the launch of the shuttle, scheduled to rendezvous with the ISS to complete the assembly of the Japanese Kibo laboratory.

Endeavour's crew of six Americans and one Canadian is scheduled to install a platform on the ISS for astronauts to conduct experiments in the vacuum of space, 350 kilometers (220 miles) above Earth's surface.

The ISS should be completed in 2010, also the target date for the retirement of the US fleet of three space shuttles.

Following Monday's scuttled launch attempt, engineers replaced the covers made from Tyvek -- a high-density synthetic material -- that protect the shuttle's nose thrusters.

One of the covers had come loose, which could have allowed rain to penetrate the thruster nozzle. The rain would have frozen when the shuttle was in orbit and could have had an impact on maneuvers, such as docking Endeavour.

In the summer, Florida weather is often unstable in the afternoon, with violent storms and heavy rains that can prevent launches.

Weather proved a thorn in NASA's side on its previous shuttle mission, in May, when Atlantis's return to earth was postponed by three days as stormy conditions forced the shuttle to touch down at its alternative landing spot in California.

The crew of the Endeavour mission includes Canadian Julie Payette, an electrical and information engineer who has been in space before and is the only woman on board.

Two other members of the crew, including Polansky, have previously traveled in space, while four of the astronauts will be on their maiden space voyage.

American aerospace engineer Tim Kopra, 46, will replace Japanese astronaut Koichi Wakata, spending several months aboard the orbiting space station.

He would be the latest addition to the permanent crew of the ISS, which is a joint collaboration between 16 different countries.

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