Nasa postponed space shuttle Discovery's launch to the International Space Station on Wednesday due to a hydrogen leak during fuelling and said it would try again on Sunday.
Sunday's liftoff was scheduled for 7.43 p.m. (2343 GMT) from the Kennedy Space Centre in Florida.
The purpose of the flight - the first of five planned for this year - is to deliver a final set of solar power panels to the space station and transport Japan's first astronaut to serve as a member of the live-aboard station crew.
Wednesday's launch attempt was called off while the shuttle was being filled with liquid hydrogen and liquid oxygen for the ride to orbit.
"Our business requires perfection and our vehicle was not perfect today," said shuttle launch director Mike Leinbach.
The leak appeared around a vent valve that funnels hydrogen gases that have boiled off during the fill-up to a disposal system on the ground, creating a potential fire hazard.
"Hydrogen gas leaking overboard is not an acceptable condition," Mr Leinbach said.
Managers expect to meet again today to discuss repair options. Nasa has until Tuesday to get Discovery off the launch pad to avoid a conflict with a Russian mission to deliver a new crew to the space station.
If Discovery cannot fly before the Soyuz arrival, Nasa will reschedule the mission for April.
A launch next week, however, will mean the shuttle crew will have to shave a few days off its planned 14-day mission.
Mike Moses, head of the shuttle mission management team, said at least one of the four spacewalks planned by the Discovery crew would be cancelled and the work rescheduled for the resident station crew members to complete.
Discovery's mission had already been delayed by a month due to safety concerns about fuel pressure valves, but after extensive testing and studies, managers cleared the ship for flight.
Wednesday's fuel leak was unrelated to the valve issue, Nasa officials said.
The main goal of Discovery's flight is to deliver a $300 million (€234.72 million) set of solar wing panels, as well as a new distiller for the station's urine recycling system.
The panels are inside a 16-tonne module that will complete the station's 11-segment exterior backbone.
The seven-man crew includes Japan's Koichi Wakata, a two-time shuttle veteran who will stay behind on the space station to serve as a flight engineer after the shuttle departs. He replaces Nasa astronaut Sandra Magnus, who has been in orbit since November.
The station, a $100 billion (€78.24 billion) project of 16 nations, has been under construction 220 miles above earth for more than a decade.
The US space agency has up to nine flights remaining to complete assembly, as well as a final servicing call to the Hubble Space Telescope, before it retires the shuttle fleet next year.