P&O Ferries has sparked outrage after sacking 800 seafarers and replacing them with cheaper agency workers.
Many of those fired were refusing to leave ships, leading to security guards with handcuffs being deployed to remove them.
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."
Labour MP Karl Turner posted a photograph on social media showing the captain of Pride of Hull addressing workers on the ship.
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".
Some of those who lost their jobs were informed of the decision on a video call. P&O has 2,200 employees remaining in the UK.
P&O Ferries said in a statement: "We have made a £100million 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."
But who are the owners behind the controversial job cuts?
P&O began operating ferries in the 1960s.
In 2019, the company was bought by Dubai-based logistics giant DP World, a “leading enabler of global trade”, for £332million.
The company has a team of more than 36,500 global employees from 103 countries and works closely with governments, shipping lines and other important constituents.
It is one of the largest port operators globally, and is owned by Dubai’s sovereign wealth fund, chaired by Sultan Ahmed bin Sulayem, who also heads Dubai’s customs authority.
In 2020, the company came under the spotlight after paying a £270million dividend to shareholders while P&O Ferries cut around 1,100 jobs due to a collapse in travel during the coronavirus pandemic.
The group owns London Gateway port – the centrepiece of the first of the controversial freeports championed by the chancellor Rishi Sunak, and established by the government last year.
According to their website, DP World signed an agreement for the Northern Transit Corridor in July 2021.
DUP MP Jim Shannon said in a statement today: "I understand that 40 per cent of the holdings of P&O Ferries is owned by a Russian company."
In a statement released at the time of the announcement, Sultan Ahmed Bin Sulayem, Group Chairman and CEO of DP World said: “DP World supports Russia's efforts to diversify trade flows between Asia and Europe.
“The Northern Transit Corridor holds out the prospect of shorter transit times between East and West.”
The agreement was signed in St Petersburg by Sultan Ahmed Bin Sulayem, and Aleksey Likhachev, Director General of Rosatom.
