The setback is the latest blow to Boeing amid a safety crisis over its 737 Max aeroplanes after a door blew off a flight over Oregon in January, which has led to new scrutiny from regulators.

David Calhoun, Boeing’s outgoing chief executive, was hounded at a US Senate hearing by families of those killed in two previous incidents in which 737 Max jets crashed. Calhoun apologised for their losses but said he was “proud” of the company’s safety record.

The ISS, which has a longer-term crew of four American and three Russian astronauts, has months’ worth of food supplies and the Starliner is able to remain docked for 45 days. If the astronauts’ return to Earth is delayed further they are likely to leave on July 2.

“I think we’re taking our extra time, given that this is a crewed vehicle, and we want to make sure that we haven’t left any stone unturned,” NASA’s Steve Stich told a press conference.

“So far, we don’t see any scenario where Starliner is not going to be able to bring Butch and Suni home.”

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Boeing’s Starliner programme is meant to provide competition to Elon Musk’s SpaceX, which has been used to ferry astronauts to the ISS since 2020. Previously, NASA had used Russian-made spacecraft.

The company has lost more than $US1.5 billion ($2.3 billion) on the project to date. It is due to complete six NASA missions under a $US5 billion contract awarded in 2014.

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