The ferry operator, bought by Dubai-based logistics giant DP World in 2019, insisted the decision to cut jobs was “very difficult but necessary” as it was “not a viable business” in its current state.
Rail, Maritime and Transport union (RMT) general secretary Mick Lynch said: “We are receiving reports that security guards at Dover are seeking to board ships with handcuffs to remove crew so they can be replaced with cheaper labour.
“We are seeking urgent legal action and are again calling for the Government to take action to stop what is fast turning into one of the most shameful acts in the history of British industrial relations.”
Mr Turner wrote: “They have support right across the city of Hull and the rest of the country, and are determined to stay on board for as long as it takes.”
Shadow transport secretary Louise Haigh said employers “cannot be given free rein to sack workers and replace them with agency staff”.
Sailings were halted on Thursday morning and would remain suspended “for the next few days”, P&O Ferries told passengers.
P&O Ferries has 2,200 employees remaining in the UK.