The GMB union has demanded a meeting with the bosses of P&O Ferries, over the future of workers after 800 workers were sacked.
The union says parent company DP World must step in or be "stripped of valuable Freeport licences".
In a statement the group said: "These sacked seafarers kept the country going during the covid pandemic, keeping shipping lines open and ensuring essential equipment, food and other supplies flowed between the UK and Europe. Management at P&O Ferries have not made these workers redundant; they have been sacked."
Maritime union Nautilus International has also urged the Transport Secretary to revoke P&O Ferries’ licences in British waters as the operator resumed services on one of its routes.
Nautilus International general secretary Mark Dickinson has written to Grant Shapps encouraging the Government to take urgent action against the ferry operator after it sacked 800 workers two days ago.
In a letter, published on Twitter, Mr Dickinson called for Mr Shapps to “hold P&O to account” in six ways, including revoking its licences to operate in British waters, pursuing “any legal option available” over how P&O handled the mass redundancies, and requesting the return of any taxpayer cash the ferry operator received during the pandemic.
Frank Macklin, GMB Organiser, said today (March 19) f: “This is a terrible day for the UK maritime industry.
“Our hearts go out to all the decent, hardworking, loyal seafaring colleagues who were sacked yesterday and their families.
“Management at P&O Ferries, under the leadership of CEO Mr Peter Heblethwaite, should hang their heads in shame.
“Mr Heblethwaite didn’t have the decency to tell seafarers, some of whom have given their lives to this company, that they were sacked. He bottled it, passing the buck to one of the company’s newest employees.
“P&O Ferries are struggling because year after year the board has made one disastrous management decision after another. Incredibly, senior managers are still in their jobs while decent seafarers get sacked.
“Parent company DP World must step in now and tackle the failed management of P&O Ferries – or they must be stripped of the licence to operate UK Freeports.
“GMB now demands a meeting with management to guarantee the future of GMB’s onshore members.”
ITV Meridian reporter, Joe Coshan, has seen a letter to remaining P&O Ferries staff, sent from CEO Peter Hebblethwaite, from one of the 800 people who lost their jobs.
In the internal email, Mr Hebblethwaite said the P&O Ferries plan to sail MS Norbank from Liverpool to Ireland today (19/03), just two days after dismissing its UK crew.
However, in frequent posts on Twitter, the company says its sailings are unable to run for the next few days.
At Dover, passengers are being told to arrive as booked but that they will be put on services by alternative carriers.
In the letter, he said the decision to sack 800 staff was "a pivotal and absolutely necessary decision for our business."
"This is a good business, but it has potential to be a golden business and these changes we've made are the start of the journey."
P&O Ferries defended the decision in a statement earlier this week, which said: “We have made a £100 million loss year-on-year, which has been covered by our parent, DP World. This is not sustainable.
“Our survival is dependent on making swift and significant changes now. Without these changes there is no future for P&O Ferries.”
P&O Ferries has been contacted for comment.