NASA said more time was needed to determine whether the problem lies with the external fuel tank valve or a gauge that was showing the valve to be open when it was supposed to be closed, but said Friday afternoon the gauge showed the valve was closed and fuelling began.

Fuel valve problems have dogged recent shuttle launches as the ageing craft and launch apparatus enters the final phase of the decades-old shuttle programme.

By late next year, NASA hopes to have completed the build-out of the orbiting International Space Station so it can retire the shuttles. In their place, a new spacecraft is being built that will be available in 2015 at the earliest.

Delayed takeoffs are common with space shuttle missions. The last shuttle to blast off, Endeavour, only did so in July after five scrubbed missions due to technical problems and bad weather.