More than 250 people work on the Pride of Bilbao
A P&O ferry service to Spain operating from Portsmouth is to be withdrawn due to "unsustainable" losses, putting 252 jobs at risk.
The Pride of Bilbao's final crossing to Bilbao - which P&O Ferries has operated for 15 years - will take place on 27 September, the firm said.
The 252 employees, including 130 agency workers, all work on the vessel. Twelve others work on shore in Bilbao.
P&O said it was talking to unions. The RMT said the route "remains viable".
Steve Todd, the RMT's national maritime secretary, said: "We believe the Portsmouth-Bilbao route remains viable and that there is no case for axing the service.
"We will be working with the company to look at options to keep the route sailing.
"In the meantime, we have cast-iron assurances that there will be no compulsory redundancies and we are in negotiations with P&O to ensure that the interests of staff are fully protected."
Crossings are expected to operate as normal up to and including the 27 September.
It is understood the company is contacting passengers with bookings beyond that date in order to make alternative arrangements or offer refunds.
The news comes after Brittany Ferries announced just before Christmas that it was withdrawing the Barfleur ferry between Poole and Cherbourg because it had been losing money since 2003.