New Delhi, May 7 (IANS): Boeing has yet again scrubbed the Starliner manned mission over an oxygen relief valve, two hours before launch, NASA announced.

The CST-100 Starliner spacecraft was expected to launch NASA astronauts Butch Wilmore and Indian-Origin Sunita Williams.

While the crew had entered the spacecraft, it faced an issue with a valve in the rocket's upper stage leading to the scrub.

“Today's #Starliner launch is scrubbed as teams evaluate an oxygen relief valve on the Centaur Stage on the Atlas V,” NASA informed in a post on X.com

“Our astronauts have exited Starliner and will return to crew quarters,” it added.

The Liftoff was targeted for 10:34 p.m. ET (0234 UTC May 7) aboard a United Launch Alliance Atlas V rocket on May 7.

“ULA Launch Director Tom Heter III has made the decision to the launch team that launch operations will not continue tonight for #AtlasV and #Starliner,” the rocket company said on X.

It is not immediately clear when Boeing and ULA might be able to make another attempt.

The setback is not the first for Boeing. After signing a contract with NASA's Commercial Crew Programme to fly operational missions to and from the space station with Starliner in 2014, it has faced several delays.

In 2019, its debut uncrewed orbital flight mission did not go as planned. The mission was completed in 2022.

Further, its crewed mission has been repeatedly delayed. On the other hand, SpaceX, which was also part of the NASA Contract in 2014, has aced about 12 flights to the International Space Station.

Starliner is aimed at carrying astronauts and cargo for future NASA missions to low Earth orbit, and beyond.