Cape Canaveral, Fla. ? With just two hours remaining in the countdown Sunday night, NASA called off the launch of space shuttle Endeavour because of an oxygen leak.

The seven astronauts had already begun boarding the shuttle when mission managers delayed the flight to the international space station by at least a week. A small leak cropped up in the system that provides breathing oxygen for the crew during launch and landing, and engineers feared it could worsen.

Earlier Sunday, NASA had loaded hundreds of thousands of gallons of fuel into Endeavour for a late-night delivery run to the space station. Liftoff had been scheduled for 11:58 p.m. CST Sunday.

The leak is believed to be somewhere in the midbody of Endeavour, beneath the payload bay. It is in one of two systems that feed oxygen into the astronauts’ pressure suits.

Endeavour was poised to carry up a fresh three-man crew to replace the current space station occupants, who have been on board since June and should have been back by now. The shuttle also holds a $390 million girder to be installed on the station by a spacewalking team.

Mission Control immediately notified the one American and two Russian space station residents about the delay in Endeavour’s launch – and their ride home. Sunday was their 158th day in orbit.

Peggy Whitson, the space station’s science officer, and two male crewmates were supposed to come home last month. Their mission was extended because of the summerlong grounding of the shuttle fleet.