P&O Ferries is to close its Liverpool-Dublin route after it was unable to secure a berth on Merseyside.
The shipping giant said the route would close later in the year and it was consulting with affected staff.
Port of Liverpool operator Peel Ports said the firm would not run the route on "expiration" of their contract.
Mick Lynch, of The National Union of Rail, Maritime and Transport Workers (RMT), said P&O could not be trusted to operate "economically vital" services.
In a statement, P&O Ferries said "extensive negotiations" had taken place in an attempt "to extend our lease... or find an alternative site for our Liverpool-Dublin service", but they had proved "unsuccessful".
"Unfortunately, despite utmost efforts by P&O Ferries to find a viable solution, no suitable alternative has been offered that would enable us to maintain the current service into 2024," a representative said.
They added that the firm was "saddened by our forced withdrawal from this route, which will reduce competition and the choice of sailings available to customers on a crossing where there is currently only one alternative operator".
Mr Lynch, the RMT's general secretary, said the government "should scrap any shipping contracts they have with P&O and begin the process of banning them from operating in UK waters, as a matter of urgency".
He said the firm "cannot be trusted to operate economically vital ferry services" and accused bosses of "blaming everyone else but themselves for their abject failures".
The P&O representative said the decision to close the route had been "forced by the port provider" and declined to respond to Mr Lynch's comments.