The ISS normally has a long-term crew of seven, but with Mr Wilmore and Ms Williams on board, nine astronauts are currently crammed into the limited space.

Ken Bowersox, NASA director of space operations, said: “At some point we have to bring Butch and Sunny home.

“While they’re up there, we have extra personnel, extra manpower, and they can do a lot more work, but they also use more consumables, more supplies, so we have to maintain that balance. And at some point we have to bring those people back home and get back to normal crew size on the ISS.

“We’re in a situation where we have several options. Not only do we have to bring a crew back on the Starliner, we could bring them back on another vehicle.”

Boeing’s Starliner project plagued by problems

The Starliner project was plagued by problems from the beginning.

After the Space Shuttle was decommissioned in 2011, NASA was forced to use the Russian Soyuz for transport to the ISS and therefore invited tenders for a new, all-American crew vehicle.

In 2014, NASA commissioned SpaceX and Boeing to develop two crew modules that could transport astronauts to the ISS.

But while SpaceX completed the contract in just six years and sent its first astronauts into space in November 2020, Boeing lagged behind for years due to ongoing technical problems.

Numerous test launches were postponed or cancelled at the last minute, and even the maiden flight carrying Mr Wilmore and Ms Williams on June 5 was temporarily postponed as engineers struggled with a “sticking valve”.

When Starliner finally launched, more helium leaks were discovered and five of the 28 engines failed while approaching the ISS.

The stranded pair were forced to perform manual maneuvers in space during a docking procedure that took an hour longer than planned.