The launch of Boeing's Starliner spacecraft first crewed test flight was called off shortly before lift-off on Monday, US space agency NASA said.
The launch was called off due to a faulty oxygen relief valve, NASA said.
US astronauts Butch Wilmore and Suni Williams had returned to crew quarters.
No new date for the test flight to the International Space Station (ISS) was announced.
Boeing plans to use the Starliner to transport astronauts to the ISS in future, providing an alternative to the SpaceX Crew Dragon capsule.
Once certified, it will carry up to four crew members to the station.
The Starliner completed an unmanned test flight to the ISS in May 2022. The spacecraft was originally due to set off for the space outpost in July 2021.
The mission has already been postponed several times, firstly due to problems after the arrival of Russian research module Nauka at the ISS, and then because of problems with Starliner's equipment.