Launch director: "Looks like the team is ready but the weather is not"
The launch of the US space shuttle Endeavour has been postponed for a fourth time, because of thunder storms in the area.
The decision was made only minutes before the shuttle was due to lift off from Cape Canaveral in Florida.
An earlier launch was called off after Nasa investigated lightning strikes around the launch pad, while two were scrapped in June over hydrogen leaks.
The launch has been rescheduled for 1851 local time (2251 GMT) on Monday.
"Looks like the team is ready but the weather is not. At this time, we are no-go," said Nasa's launch director, Pete Nickolenko.
The lift-off had been timed for 1913 local time (2313 GMT) from the Kennedy Space Center, some 24 hours later than planned.
Forecasters had said there was only a 30% chance of a weather-related delay.
It came after engineers spent Saturday checking over the shuttle to ensure that there was no damage to its electrical systems. Lightning had struck 11 times near the launch pad on Friday.
Two previous launch attempts in June were also scrapped because of a potentially hazardous leak in a hydrogen vent line.
The Endeavour crew is set to install an external platform on Japan's space station lab, Kibo.
The orbiter is taking a seven-strong crew into space, made up of six Americans and one Canadian - Julie Payette - who will operate the shuttle's robotic arm during the mission.
Their arrival will bring the total crew on the outpost to 13 - a record for the ISS.
The shuttle itself was not hit by the lightning strikes
During five spacewalks, a platform will be added to the Japanese lab complex, which can be used for experiments that require materials to be exposed to the harsh environment of space.
In addition, Endeavour will deliver a new long-stay US crew member, Tim Kopra, to the ISS and bring back Japan's Koichi Wakata, who has lived aboard the platform for more than three months.
The space station, now about the size of a four-bedroom house, has been under construction for more than a decade.
When it launches, Endeavour will make the 127th space shuttle flight, the 29th to the station, the 23rd for Endeavour and the third in 2009.
Seven further flights to the platform remain before the shuttles retire in 2010.