NASA engineers have found a 13 centimetre long crack in the insulating foam on the large external fuel tank of the shuttle Discovery, a spokesman for the space agency said on Monday.
NASA made the announcement one day before before Discovery was due to lift off with seven astronauts on a crucial mission for US space ambitions.
Officials said it is unclear how the finding may affect NASA's planned launch of the shuttle on Tuesday.
"We don't know if it's a problem or not (for the launch)," said NASA spokesman George Diller.
"If we decide we have to do something about it we probably cannot launch tomorrow," Diller said.
He said the crack was about one-eighth to one-quarter of an inch deep and five inches long, and that it had been found in the foam on a bracket that holds an oxygen feed line.
The discovery of the crack comes after storms forced NASA to scuttle two weekend launch attempts, forcing the space agency and launch crew to take a break today, allowing the fuel cells that power the shuttle in orbit to be recharged.
Discovery's five men and two women astronauts are due to fly to the International Space Station (ISS) on a mission aimed at improving shuttle safety.
Their space mission comes three years after the shuttle Columbia disintegrated while returning to Earth.
During the planned 12-day flight, the astronauts are meant to test new safety procedures and wil deliver vital equipment and supplies to the ISS.