Discovery's engines had been due to start on Saturday afternoon at 3:49 pm local time (1949 GMT) from the Kennedy Space Center.
The Shuttle’s crew of five men and two women are heading to the International Space Station (ISS).
NASA said initial preparations were going ahead smoothly despite earlier concerns from engineers about the safety of foam insulations that had come off the shuttle's fuel tank during its previous launch last year.
Space officials went ahead with the countdown for the flight only the second since the Febraury 2003 Columbia disaster.
But Kathy Winters of NASA's meteorological service said gathering storm clouds meant there was a 60-percent chance that the flight would be postponed.
With the Columbia disaster still fresh in the memories of NASA personnel, another catastrophe could put a permanent end to the 25-year-old shuttle program.
The 115th shuttle flight will be headed by Commander Steven Lindsey. His crew includes co-pilot Mark Kelly, and mission specialists Michael Fossum, Lisa Nowak, Stephanie Wilson, Piers Sellers and the European Space Agency's Thomas Reiter of Germany.
Safety fears
The astronauts will inspect the shuttle for any damage during the mission, STS-121, and test new equipment and procedures to increase safety.
The crew will also deliver critical cargo for the ISS, while Fossum and
Sellers will conduct two or three spacewalks to do maintenance work on the space station.
Reiter will then remain in the ISS, joining American astronaut Jeffrey Williams and Russian cosmonaut Pavel Vinogradov.
NASA gave the green light for the launch despite calls from its top safety and engineering officials for a delay.
NASA’s safety experts wanted to ensure that potentially damaging foam insulation did not peel off the orbiter's external fuel tank again.
Chief safety officer Bryan O'Connor and chief engineer Chris Scolese had asked for a six month delay to make more modifications to insulating foam on the external fuel tank.
Columbia's wing was struck by a piece of foam that fell off the tank during takeoff, dooming the spacecraft as it returned to earth with seven astronauts aboard.
After a flight readiness review, NASA administrator Michael Griffin announced that this launch would go ahead despite the dissenting views expressed by O'Connor and Scolese.
The launch has a July 1-19 window.
Without a shuttle, the United States would have to rely on Russia's Soyuz spacecraft to service the ISS.
NASA is already looking beyond the shuttle program with plans to build a Crew Exploration Vehicle (CEV) by 2014.