NASA reported that technicians noticed an anomaly in fuel cell No. 1 before tanking this morning. The fuel cells, which provide power to the orbiter, experienced a drop in voltage and then a spike, causing NASA to decide to scrub the launch so that they continue to troubleshoot before having to tank.
Officials say there will be no press briefing until after the 1 p.m. mission management team meeting.
The shuttle, which is on a construction mission to the international space station, was already delayed by the Columbia shuttle accident, continued safety problems after Discovery's flight in 2005, a lightning strike in August and the threat of Tropical Storm Ernesto.
If Atlantis doesn't get in the air this week, the next chance probably won't come until late October.
The shuttle was to deliver a 35,000-pound, $372 million addition to the half-built international space station during an 11-day mission. Four astronauts will take three spacewalks to resume construction on the orbiting lab, the first such work in 3 1/2 years after the Columbia disintegrated while returning to Earth.
This mission to deliver two girders crucial to the space station's continued expansion must be done before the final 14 shuttle flights.
The Russians plan to launch a Soyuz capsule on Sept. 18 ferrying two new station crew members and the space station's first female tourist, Dallas-area entrepreneur Anousheh Ansari. Officials with both space agencies wanted to avoid the shuttle and Soyuz meeting at the station, fearing a traffic jam.
Atlantis' mission will be the first since late 2002 to expand the space station. The last two flights were tests evaluating a redesign of the external fuel tank, whose falling foam was blamed for the Columbia accident.
FOXNews.com's Matthew Ruiz and the Associated Press contributed to this report.