Endeavour was to carry seven astronauts to the space station
The launch of space shuttle Endeavour has been cancelled due to a hydrogen leak, Nasa officials have announced.
Endeavour was due to take off from Cape Canaveral with seven astronauts at 0717 local time (1117 GMT), but launch was cancelled several hours beforehand.
The problem was discovered during fuelling, before the astronauts had donned their spacesuits.
The shuttle was to deliver equipment for space experiments, as well as drop off a new station crew member.
Mission controllers are expected to meet on Saturday to decide when to reschedule the launch.
The leak was found on the Ground Umbilical Carrier Plate, which is attached to the shuttle's external tank.
The leak is similar to one which grounded the Endeavour's sister shuttle Discovery in March.
When it does launch, the ship will take three days to reach the International Space Station (ISS), where six astronauts work.
With the shuttle's seven crewmembers, it will mean there will then be 13 individuals at the platform - a new record.
Endeavour will carry its usual complement of seven astronauts
Endeavour is due to deliver the final components of Japan's Kibo laboratory.
During five scheduled spacewalks, an external platform will be added to the lab that will enable experiments to be performed that require materials to be exposed to the harsh environment of space.
Endeavour astronauts also need to fit equipment to the exterior of the platform such as batteries and a spare space-to-ground antenna.
Endeavour will deliver a new crew member (Tim Kopra) to the ISS and bring back another (Koichi Wakata) who has lived aboard the platform for more than three months.
When it does lift off, Endeavour will be making the 127th space shuttle flight, and the 29th to the station.