Heavy clouds near the Kennedy Space Center forced NASA to call off Sunday's launch of the space shuttle Endeavour on a mission to deliver a Japanese-built porch to the International Space Station.

The launch was rescheduled for 6:51 p.m. EDT on Monday.

It was the fourth delay of Endeavour's flight. Two launch attempts last month were scuttled by hydrogen fuel leaks and a third try on Saturday ended when NASA ordered checks of the shuttle's electrical systems following a spate of lightning strikes at the launch pad on Friday.

The clouds moved into the area less than an hour before Endeavour's scheduled 7:13 p.m. EDT launch, with the seven astronauts strapped into their seats and the ship loaded with 500,000 gallons (1.9 million liters) of fuel to blast it into orbit.

Endeavour's primary cargo is a porch for Japan's $2.4 billion Kibo laboratory complex on the space station. The platform can be used to expose experiments to the open environment of space.

A small robotic crane will be able to retrieve and service experiments on the platform without time-consuming and potentially dangerous spacewalks by station crewmembers.

NASA launched the first section of the elaborate three-part complex in March 2008 and the second section two month later.

The porch is scheduled to be installed during the first of five spacewalks planned during Endeavour's 12-day stay at the outpost, a $100 billion project of 16 nations.

The shuttle also is carrying replacement batteries for the solar-powered station and spare parts that will be needed to keep the outpost operational after the U.S. shuttle fleet is retired next year.

The space station has been under construction 225 miles above Earth for more than a decade. It consists of nearly 26,000 cubic feet (735 cubic meters) of pressurized space, about as much room as a typical four-bedroom house.

NASA plans to complete construction of the station by September 30, 2010, and retire its three space shuttles. The agency then plans to ramp up development of new capsule-style spaceships that can carry astronauts to the moon as well as to the station.

One of the Endeavour crew's first tasks will be to transfer astronaut Timothy Kopra to the space station crew. He replaces Japan's Koichi Wakata, who has been aboard the outpost since March.

Wakata will return to Earth with commander Mark Polansky and the rest of the Endeavour crew -- pilot Doug Hurley, David Wolf, Christopher Cassidy, Tom Marshburn and Canadian Julie Payette -- on July 28.

(Editing by Jim Loney and John O'Callaghan)

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