P&O Ferries crew members are refusing to leave their vessels after sailings were halted on Thursday.
The DP World-owned company ordered all vessels back to UK ports as media and unions reported all UK seafarers had been made redundant immediately.
The plan is to save money by bringing in cheaper agency workers to secure the ferry operator’s long-term future.
Private security officers have been sent on to one ship docked at Larne Harbour in Northern Ireland to remove staff on board, according to the RMT union.
“We are digging in for the long-haul. We are determined to fight,” RMT spokesman Geoff Martin said.
A seafaring P&O employee told the BBC his colleagues on board have refused to disembark and are instead “in their cabins refusing to work”.
There have been reports of coaches of agency seafarers waiting in ports ready to get the vessels running again later on Thursday.
Crew union Nautilus International’s general secretary Mark Dickinson called the redundancies a “betrayal” of British workers.
Referring to furlough payments during Covid-19 lockdowns, he added: “It is nothing short of scandalous, given that this Dubai-owned company received millions of pounds of British taxpayers’ money during the pandemic.”
There was no notice and no consultation by P&O, Dickinson said.
He promised that “the full resources” of the union stand ready to defend its members.
The TSSA union condemned a plan to make seafarers apply to an agency reportedly set up by P&O to get their jobs back.
Manuel Cortes, TSSA general secretary, said: “This is absolutely despicable behaviour from P&O, designed to reduce pay and worsen terms and conditions for their staff. They should be ashamed of themselves, treating loyal and hardworking staff like this.”
The union said that in any civilised country these actions would not only be unlawful but punishable in the harshest possible terms.
“Sadly, I doubt the Tory government will lift even their little finger to ensure this happens,” Cortes added.
Nautilus and the RMT are advising members to stay on board to protect their interests.
P&O Ferries employs nearly 4,000 staff. It cut 1,100 jobs during the pandemic.
