The space shuttle Endeavour's launch, which had been set for Saturday with Canadian astronaut Julie Payette on board, has been postponed. Here it is shown blasting off on a previous mission. (Marta Lavandier/Associated Press)

The planned Saturday launch of space shuttle Endeavour, set to carry seven astronauts including Canadian Julie Payette, was postponed because of a hydrogen leak — delaying a possible flight until Wednesday at the earliest.

The leak was discovered while the space shuttle was being fuelled. Endeavour had been scheduled to launch from the Kennedy Space Center in Florida at 7:17 a.m. ET on Saturday for a mission to the International Space Station.

The problem is similar to one the space agency faced in March during the launch countdown for space shuttle Discovery.

Launch director Mike Leinbach said the leak, at a vent line hookup on the fuel tank, was significant. Hydrogen gas is extremely volatile and can burn in large enough quantities, he noted.

Canadian astronaut Chris Hadfield likened the delay to fixing a plumbing problem.

"But from an astronaut's point of view, this is just absolutely ops [operations] normal. It is really hard to do this. Sometime you have to fix something and it just takes a little time, and that's OK."

NASA has yet to decide when Endeavour will launch, but Wednesday is the next possible launch date.

'Wait a few days': Payette

"I am sorry for those of you who have travelled to Florida to see the launch and hope you will be able to wait a few days for our next launch opportunity," Payette wrote in a message posted on the Canadian Space Agency's website.

"Our crew is taking things in stride and will be ready to go when called."

Payette was the first Canadian to visit the space station in 1999. In the coming mission, she will join Canada's Robert Thirsk, who is already aboard.

This would be the first time two Canadians have been in space at once.

Thirsk has completed one month of a six-month stay on the space station, marking the first time a Canadian has participated in a long-duration space flight.

Payette is set to play an important role in the mission. As a flight engineer, she'll be operating three robotic arms on the station — the Canadarm 1, the Canadarm 2 and the new arm that is to be installed on the Kibo laboratory.

The shuttle is expected to take the astronauts and a piece of the station's Japanese-designed Kibo laboratory to the space station.

During the 16-day mission, the astronauts will work with the crew of six already on the station to install a robotic arm and two platforms on the Kibo laboratory and perform five space walks.

One platform will remain permanently attached to the station and will function like a porch for experiments that require direct exposure to space.

The other platform will be used for experiments to be performed during the mission. It's designed to return to Earth with the shuttle.

With files from The Associated Press