NASA postponed the planned launch of the space shuttle Endeavour today due to a hydrogen leak that developed during fueling, a space agency official said.
Endeavour and its seven-member crew had been scheduled to blast off at 1117 GMT from the Kennedy Space Center in Florida on a mission to deliver the last part of a Japanese laboratory to the International Space Station.
NASA spokesman Allard Beutel said crews found a gaseous hydrogen leak in ground support equipment while the shuttle was being fueled with a 1.9 million litres of liquid oxygen and liquid hydrogen.
Endeavour will carry a Japanese-built porch that will be installed on the space station's $2.4 billion Kibo complex, Japan's primary contribution to the $100 billion orbiting research outpost.
The lab includes a robot arm and a small airlock so science experiments can be installed outside the station and retrieved without the need for spacewalks by station or shuttle crew members.
Endeavour will also carry laboratory racks, experiments, supplies and dozens of spare parts, which will be stored aboard the outpost in preparation for when the shuttle is no longer flying.
NASA has eight shuttle missions remaining to complete construction of the space station. It plans to retire the three-ship fleet at the end of 2010.
The Endeavour crew plans to spend 12 days at the station and conduct five spacewalks to install the porch, replace batteries and tackle a long list of maintenance tasks.
The mission is NASA's first since the station's live-aboard crew was expanded to six members from three last month.
When the shuttle crew arrives, it will bring the total number of people aboard the space station to a record 13.
The combined crew includes two Canadian astronauts -- station flight engineer Robert Thirsk and Endeavour astronaut Julie Payette -- Japan's Koichi Wakata, Europe's Frank De Winne, Russian cosmonauts Gennady Padalka and Roman Romanenko and seven U.S. astronauts.
Wakata will be returning home with the shuttle crew after a three-month stay on the station. His replacement is NASA astronaut Timothy Kopra, one of Endeavour's four rookie fliers.