Cancellation of inaugural crewed space mission comes as Boeing is under fire over safety record at its aviation arm.

The decision to call off the launch on Monday came two hours before the scheduled liftoff and about an hour after two NASA astronauts had strapped into the spacecraft.

The postponement, blamed on a problem with a valve in the Atlas V rocket, was announced during a live NASA webcast.

“Standing down on tonight’s attempt to launch,” NASA chief Bill Nelson said in a post on X.

“As I’ve said before, @NASA’s first priority is safety. We go when we’re ready.”

It was not immediately clear how long it would take to address the problem, but the next available launch windows for the launch are Tuesday, Thursday and Friday nights.

Standing down on tonight’s attempt to launch #Starliner. As I’ve said before, @NASA’s first priority is safety. We go when we’re ready. https://t.co/KIasomZG66

— Bill Nelson (@SenBillNelson) May 7, 2024

The Starliner had been due to transport NASA astronauts Butch Wilmore and Suni Williams to the International Space Station (ISS), where they would have spent a week before returning to Earth.

The Starliner’s inaugural voyage to the ISS has been closely watched as a sign of Boeing’s ability to rival Elon Musk’s SpaceX for NASA contracts.

NASA in 2014 awarded multibillion-dollar contracts to Boeing and SpaceX to develop space capsules for the space agency to ferry astronauts and cargo into space.

The contracts marked the start of a shift by NASA towards public-private partnerships following the end of its space shuttle program.

SpaceX’s Dragon successfully transported astronauts to the ISS in 2020, marking the first time NASA astronauts had launched from US soil in a commercially-built spacecraft.

Starliner flew its first uncrewed mission to ISS in 2022, after an unsuccessful attempt three years earlier.

Monday’s launch cancellation comes at a difficult time for Boeing as the company grapples with several probes into alleged safety lapses at its aviation division.