"It will not happen before Monday," he told journalists at the Kennedy Space Center where the shuttle had been due to lift off at 3:51 p.m..

But he said it remained unclear how quickly the problem could be fixed. "We don't know how long it will take, it depends on the nature of what we have to do," Griffin said.

He pointed out such delays are not uncommon. "I had one mission that scrubbed 14 times on the pad, this is nothing."

A faulty hydrogen fuel sensor that could have prevented the shuttle from reaching its full power during ascent into orbit caused NASA managers to call off the launch two hours and 20 minutes before Discovery was due to lift off.

The sensor is one of four designed to transmit data to computers on the levels of hydrogen remaining in the main tank, to determine when engines should be shut off during the ascent into orbit.

The sensor did not respond during tests conducted a few hours before the planned launch at the NASA facility on Florida's seashore.

"We require all four sensors to work correctly before launch," Griffin said.

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