P&O Ferries has stopped all its operations and has laid off more than 800 workers.
The company – which plies the sea route between Dover and Calais – has ordered all its ships to return to port.
However, some have refused to do so.
The staff were dismissed in a Zoom video message from bosses.
They were told they were being “terminated immediately” – to be replaced by agency staff.
P&O reports it has lost more than £100 million over the last year.
In a pre-recorded video announcing mass redundancies, staff were told:
“The company has made the decision that its vessels going forward will be primarily crewed by a third-party crew provider
“Therefore, I am sorry to inform you that this means your employment is terminated with immediate effect on the grounds of redundancy.
“Your final day of employment is today.”
Gary Jackson, who worked on the ferry Pride of Hull, said;
“We’ve still not received any detail further on what they will offer.
“We can see from the ship two vans, one with agency staff and the other with what we believe are security staff to remove us… and that’s why the captain here lifted the gangway.”
One woman, who did not want to be named, said her husband and son lost their jobs on Dover ferries overnight.
“When P&O suspended sailings this morning crews were waiting on the ships with no idea what was happening – even the captains didn’t know.
“Then pictures emerged of three coach loads of foreign workers waiting at ports to take their jobs.
“They were already wearing P&O uniforms – and the staff still hadn’t been told they were being let go. They were told they had five minutes to get their stuff and get off the ships.
“These guys are week on week off workers who have called these ships home for half of the year every year.
“They have personal effects in lockers which they can’t get to – clothes, bedding, photographs and in some cases financial documents which will be taken out and thrown.”
The RMT union said the issue was “fast turning into one of the most shameful acts in the history of British industrial relations”.
The general secretary of the RMT, Mick Lynch, said:
“We have instructed our members to remain onboard and are demanding our members across P&O’s UK operations are protected, and that the Secretary of State intervenes to save UK seafarers from the dole queue.”