The ISS usually holds a long-term crew of seven, but with Mr Wilmore and Ms Williams on board there are currently nine astronauts crammed into the limited facilities.
Ken Bowersox, Nasa’s director of space operations, said: “At some point we need to bring Butch and Sunny home.
“While they’re up there we have extra crew, we have extra hands and they can do a lot more work but they’re also using up more consumables more supplies so we have to maintain that balance. And at some point we need to bring bring those folks home and get back to a normal crew size on the ISS.
“We are in a situation where we’ve got multiple options. We don’t just have to bring a crew back on Starliner, we could bring them back on another vehicle.”
Boeing Starliner project plagued by problems
The Starliner project has been beset with problems from the outset.
After the space shuttle was decommissioned in 2011, Nasa was forced to use the Russian Soyuz to travel to the ISS, and so invited bids for a new all-American crew vehicle.
In 2014, Nasa chose SpaceX and Boeing to design two crew modules capable of ferrying astronauts to the ISS,
But while SpaceX completed the brief in just six years, launching its first astronauts in November 2020, Boeing lagged years behind amid ongoing technical issues.
Numerous test launches were delayed or scrapped at the last minute, and even the inaugural flight with Mr Wilmore and Ms Williams on June 5 was temporarily postponed while engineers dealt with a “sticky valve”.
When Starliner eventually launched, more helium leaks were detected and five of the 28 thrusters failed as it approached the ISS.
The stuck pair were forced to undertake manual manoeuvres in space in a docking sequence that took an hour longer than planned.