Endeavour's launch was postponed Friday and won't proceed for at least three days due to a technical issue, NASA says.
The space shuttle's final blastoff had been scheduled for 3:47 p.m. ET Friday, but will now go ahead no earlier than 2:33 p.m. Monday, NASA said.
NASA announced shortly after noon that the launch would not go ahead Friday, citing the failure of the circuits of two heaters on the shuttle's auxiliary power unit.
"The engineering team did not understand how this problem occurred and did not feel comfortable proceeding with a launch attempt," NASA spokesman George Diller said in a statement posted on Endeavour's launch blog.
The units provide hydraulic power for the shuttle's landing gear and brakes, and allow for precise control of the shuttle's orientation and that of its rocket engine. The heaters are needed to keep the power units' hydrazine fuel from freezing while the shuttle is in orbit.
NASA said the launch would be delayed at least 72 hours, up from an initial estimate of 48 hours, while engineers assess the issue.
Later in the day, NASA gave more information about the timing.
"Once the external fuel tank is drained and safe to work around, which takes about 24 hours, technicians will be able to access the aft of the shuttle. By Saturday night, they should be able to start the troubleshooting process," NASA said in a release.
The U.S. space agency said it would hold a news conference at 4 p.m. Friday about its decision.
Stats on the shuttle Endeavour prior to its final launch:
Kilometres travelled: 166,003,248
Time in space: 280 days, 9 hours, 39 minutes, 44 seconds
Crew members: 148
International Space Station dockings: 10
The announcement came after the shuttle's six-member crew, commanded by Capt. Mark Kelly, had already put on orange launch and re-entry suits, and headed toward the launch pad at Kennedy Space Center in Florida.
Their vehicle turned around and returned to the building that houses the crew quarters.
Later in the day, the shuttle's external fuel tank will have to be drained of 1.9 million litres of liquid hydrogen and oxygen fuel with which it had been filled earlier in the morning.
Endeavour is heading to the International Space Station on a 14-day mission to deliver the Alpha Magnetic Spectrometer (AMS), a particle physics detector that scientists hope will help lead to the understanding of the origin of the universe.
The shuttle will also deliver spare parts, including some specifically for Dextre, a Canadian Space Agency robot that rides on the end of the space station's Canadarm2 robotic arm. Dextre performs maintenance and repair work outside the station, such as changing batteries and cameras.
Thousands of people were expected to attend the high-profile launch, including Kelly's wife, Gabrielle Giffords, Democratic member of the House of Representatives, who is recovering after being shot in the head in January.
U.S. President Barack Obama and his family had hoped to watch the launch and went to Cape Canaveral despite the cancellation. They met with Giffords for about 10 minutes.
The president told Endeavour's crew he is still hoping to get back to Florida to see the launch.
"One more chance, we may be able to get down here," Obama said.
"It's a priority for us," Michelle Obama added.
Following the launch, Endeavour will be put on display at the California Science Center in Los Angeles. Its Canadarm robotic arm is expected to be returned to Canada.