The craft had been due to blast off late on Friday, Florida time, from Cape Canaveral, after the launch had already been postponed for more than a week due to a problem with the shuttle's oxygen supply.

The orbiter is scheduled to carry seven astronauts to the International Space Station (ISS), including the new resident crew, Expedition Six.

Two of the astronauts are last-minute replacements for crew members who had to be pulled off the mission on medical grounds - the first time the US space agency Nasa has had to switch crews since the ill-fated Apollo 13 voyage.

The conditions at the back-up sites at Moron, in southern Spain, and Zaragoza, in the north, have to be good enough to enable the shuttle to land in the event of an emergency.

Nasa officials said they would attempt to launch on Saturday, weather permitting.

Oxygen leak

The 16th shuttle mission to the ISS was to have lifted off on Monday, 11 November, but with three hours to go the mission was stood down.

The shuttle had already been loaded with more than 1.9 million litres (500,000 gallons) of rocket fuel. The problem was a leak in the system that provides oxygen to the crew's helmets.

The shuttle's robot arm was also damaged in an accident; workers rammed a platform into the 15-metre (50 feet) arm.

After more than a week of repairs, Nasa says the shuttle is now ready for launch.

Endeavour will take the new resident crew to the ISS and bring the current one back to Earth.

Crew replacements

Other members of the shuttle team will attach another huge beam, or truss, to the ISS using the station's robotic arm. The truss will form part of the station's cooling system.

Pilot Paul Lockhart is a last-minute replacement

It will also increase the length of the main section of the ISS by 14 metres.

Two of the original mission crew, Donald Thomas and Endeavour pilot Christopher "Gus" Loria, were replaced in the summer after both suffered medical problems.

Shuttle pilot Paul Lockhart and Donald Petit will replace them.

Donald Petit will join Commander Ken Bowersox and Flight Engineer Nikolai Budarin, who together make up the sixth resident crew on the ISS.

Expedition Five will return to Earth with Endeavour. They have been on the platform for than five months on the ISS.