The space agency did not immediately set a new launch date.

The decision came with less than two and a half hours left in the countdown.

Up until then, thunderstorms appeared to be the only obstacle to an on-time lift-off.

Launch control said it will take some time to figure out the problem.

The problem was with one of the four engine cut-off sensors, which are responsible for making sure the spacecraft's engines shut down at the proper point during the ascent.

Nasa said it appeared that the sensor was showing a low fuel level, even though the tank was full.

The sensors "for some reason did not behave today and so we're going to have to scrub this launch attempt," launch director Mike Leinbach told the launch team. "So appreciate all we've been through together, but this one is not going to result in a launch attempt today."

Launch control said it will take some time to figure out the problem.