P&O Ferries has said it will pay out more than £36.5m in compensation to workers who were sacked last week.
The company said it believed it was the largest package of its kind in the British marine sector.
Unions called P&O’s statement “disgusting” for trying to “justify one of the most shameful acts by any employer in recent history”.
The ferry operator fired 800 workers on the spot via a pre-recorded video call and suspended all of its services on Thursday.
Security guards boarded ships with handcuffs to remove the sacked crews, it was alleged.
The Rail, Maritime and Transport union (RMT) said sacked seafarers had been told to sign non-disclosure agreements in order to qualify for the redundancy pay-out.
P&O Ferries said 575 of the 786 seafarers affected are in discussions to progress with severance offers.
The company also said the sacked workers are being given support to find a new job at sea or onshore.
Union Nautilus International accused P&O Ferries of trying to “split union solidarity and influence the public by giving details of compensation scheme and how many crew have signed a package”.
RMT general secretary Mick Lynch said: “The way that the package has been structured is pure blackmail and threats– that if staff do not sign up and give away their jobs and their legal right to take the company to an employment tribunal they will receive a fraction of the amount put to them.
“The actions of P&O demonstrate the weakness of employment law and protections in the UK. P&O have flagrantly breached the law and abandoned any standards of workplace decency.”
A protest has been organised by union members at the P&O terminal at Cairnryan on Wednesday.
P&O Ferries boss Peter Hebblethwaite has been summoned to appear before MPs on Thursday morning to explain why he decide to sack his UK crews.