BBC World is providing live coverage of Friday's countdown at 0836 GMT.

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The BBC's Peter Biles: "Minutes before the launch, an alarm sounded"

The countdown was restarted but the problem could not be resolved and Nasa eventually abandoned the launch with less than twenty seconds left on the clock.

"We did not pick up the countdown in time to make the launch window," Shuttle launch control said. "We have stopped the countdown at 19 seconds. The vehicle is in a safe mode."

Nasa had no choice but to stop the countdown

The space vehicle should have gone up just before 0300 EST (0900 GMT. It will now try again on Friday morning at about 0336 EST (0836 GMT).

Because of the complex manoeuvres the shuttle must perform in orbit to catch and dock with the first ISS component launched last month, Endeavour has a very narrow window in which to take off - just ten minutes each day.

The Shuttle can go up any day until Tuesday, 8 December, when a scheduled Delta rocket launch would keep it grounded. If that happens, Nasa may decide to delay the mission until after Christmas.

Master alarm

The master alarm is a pair of rectangular red lights in the shuttle cockpit. Endeavour's pilots reported that the alarm went off at the time the shuttle's hydraulic power units were turned on.

Shuttle launch control explains what went wrong

"We progressed down to the count at five minutes where we start up our auxiliary power units which provide us with hydraulic power to operate our flight controls, main engine valves and several other mechanical systems on the vehicle", said Donald McMonagle, one of Nasa's managers.

"After the start of those [units], we take them from a low pressure to a high pressure configuration and it was at that point that we got a master alarm that we had not seen before in this configuration."

Hydraulic power units appeared to trigger the alarm

When the six astronauts crawled out of the shuttle an hour later, they threw up their hands. "We'll try again another time," said commander Robert Cabana. He held up his left thumb and index finger a centimetre apart. "We were that close."

Launch director Ralph Roe, while disappointed, was convinced his team did the right thing. He said they would try again Friday. "We want to err on the conservative side," Roe said. "We don't want to launch with something we don't understand."

Zarya module

Endeavour's flight will be the first manned mission in the ISS project. Its task is to deliver the next component for the ISS - a 13-tonne, six-sided connecting hub called Unity.

Watch Nasa's simulation of the Endeavour mission

It will be joined to the Zarya module launched last month by a Russian Proton rocket from the Baikonur cosmodrome in Kazakhstan. Two astronauts will make three, six-hour-long spacewalks to connect the modules. They will be working some 340 kilometres (212 miles) above Earth.

The Unity module will become the primary docking port for future shuttle missions during construction of the multi-billion-dollar space station.

The BBC's Leo Enright: "One more step on the road to Mars"

Three US Shuttles and two unmanned Russian rockets will undertake 45 missions to launch and assemble more than 100 components before the station is fully operational in 2004.