Low clouds descended over the launch pad about two hours before the planned 4:39 a.m. local time liftoff.

The conditions violated safety rules for the flight.

Endeavour and its six astronauts had been scheduled to take off before daybreak on a 13-day mission to install two new modules on the International Space Station.

The next launch opportunity would be on Monday at 4:14 a.m. local time.

The shuttle is set to deliver two modules for the station.

They are a connecting node named Tranquility, and a seven-sided viewing cupola for the crew to oversee robotics and gaze at Earth.

Endeavour´s crew of five men and one woman plan to conduct three spacewalks to hook up the new gear.

Five shuttle flights remain to complete assembly of the $100 billion dollar International Space Station before the U.S. fleet is retired later this year.

U.S. President Barack Obama has announced plans to cancel the station´s follow-on program called Constellation.

That program was aimed at returning U.S. astronauts to the moon in the 2020s.

Instead, Obama wants NASA to seed development of commercial space taxis to ferry crew members to and from the station.  

He also wants NASA to develop technologies to prepare for eventual human missions to other destinations in the solar system.