Launch officials said they would try again on Tuesday after giving the work force a day of rest and a chance to replenish the shuttle's on-board fuel. The weather was expected to improve by Tuesday, although rain was still in the forecast.

"We've concluded that we're not going to have a chance to launch today," launch director Mike Leinbach announced to his team.

Replying from the cockpit, shuttle commander Steven Lindsey said: "Looking out the window it doesn't look good today, and we think that's a great plan."

He noted that Tuesday - Independence Day in the US - would be "a good day to launch".

Lindsey and the six other astronauts had boarded the spaceship an hour earlier for what would have been the second shuttle flight since the 2003 Columbia disaster. The countdown was halted with more than an hour remaining, much earlier than Saturday's postponement.

Storms brewing

Astronaut Mark Kelly exits the
Discovery at Kennedy Space Centre

The afternoon sky was darker than on Saturday and left Nasa with little choice but to call off the launch to the international space station.

Thunderstorms were moving in quickly from the west, and lightning was detected within a few miles of the launch pad.

The delays cost Nasa more than $2 million in overtime pay and fuel costs.

Once in orbit, Discovery's seven-member crew will test shuttle inspection and repair techniques, bring supplies and equipment to the space station and deliver the European Space Agency's Thomas Reiter for a six-month stay aboard the orbiting outpost.