The shuttle is on a mission to start construction work on the International Space Station (ISS).

Scientists at Cape Canaveral said the cockpit alarm which caused Thursday's blast-off to be aborted posed no threat to the mission. They said that if the alarm sounded again under similar circumstances, the launch would still proceed.

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

Replenishment of the craft's giant fuel tanks is said to be proceeding smoothly.

19 seconds

On the first launch attempt, a master alarm sounded in Endeavour's cockpit with just four minutes left on the countdown clock and launch control was forced to suspend the blast-off from the Cape Canaveral, Florida.

Hydraulic power units appeared to trigger the 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.

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.

Watch Nasa's simulation of the Endeavour mission

"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."

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.

Nasa had no choice but to stop the countdown

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.

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.

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

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.