P&O Ferries has been told it must pay employees at least the minimum wage or face being banned from UK ports.
The firm, which operates routes from Dublin and Larne ports, sacked 800 staff earlier this month.
It plans to hire lower-paid agency workers instead.
British transport secretary's written a letter to the company's chief executive saying he intends to "block" that outcome.
Mick Lynch, from the RMT Union, says workers' rights must be protected
"We've got a constructive dialogue with the Government officials from the transport department and we're looking at constructing a package of measures this week that will assist in the process of bringing P&O to account, and also restructuring the, the British shipping sector."
