P&O Ferries have come under fire as it said it offered 800 redundant staff £36.5m in total - with around 40 getting more than £100,000 each and denied breaking the law.
Maritime union Nautilus said the "shameless" action showed that the ferry firm had tried to "buy its way out of the legal predicament".
In the UK, when 100 or more employees are proposed to be made dismissed, a 45-day consultation has to take place and collective redundancy rules apply and the business secretary has to be notified.
A failure to comply is a criminal liability.
The details emerged as the Prime Minister said that it looked as though the ferry group owned by Dubai-based DP World had broken the law as laid out in the Trade Union and Labour Relations Act 1992 and that the government will take legal action against them.
He said they will be encouraging workers to take action under the 1996 Employment Act.
P&O has denied that it broke the law when it sacked the workers without warning last week.
But unions said the compensation package being offered was "pure blackmail and threats".
The last P&O social media statement about its services came six days ago.
A Nautilus spokesman said: "It has taken the company five days to come out in the open with an explanation – of sorts, but not an explanation of its outrageous and illegal actions but an admission of guilt; guilt that it intends to attempt to assuage with cash. P&O Ferries shameless actions now extend to trying to buy its way out of the legal predicament, exposed by the unions, of failing in its obligation to report to the Secretary of State and to consult with the recognised trades unions.
"P&O Ferries are claiming to be offering the ‘largest compensation package in the British maritime sector’– a company in such desperate financial circumstances that it is prepared to spend £36.6m on a fake redundancy.
"Let's be clear, this statement is confirmation that P&O Ferries believes that with enough money it has no need to follow the laws of this country or be hindered by the seemingly shallow epithets of its parent’s ‘ethical business standards’. Instead, it intends to bully our maritime professionals into signing settlement agreements to buy their silence.
"Nautilus members have been put under intense pressure by P&O Ferries management to accept these redundancy terms whilst threatening them with withdrawal of the offer made if they talk to the press or demonstrate against the company. We are not surprised that many of our members are voting with their feet and leaving. We seriously doubt many will return for reduced terms and conditions and the promise of a bonus in 12 months from a Maltese [agency recruitment] outfit that didn’t exist five weeks ago."
P&O said some employees are set to get 91 weeks' pay and the chance of new employment, and no employee would receive less than £15,000.
Nautilus general secretary Mark Dickinson said “We demand that P&O Ferries engages with the recognised unions, reinstate all staff who were sacked, and discuss our serious safety concerns about how the company now plans to operate the ships.”
“Only then will P&O Ferries be able to live up to its heritage, contribute to the achievement of the government’s Maritime 2050 strategy and recover the damage it has done, not only to the nations standing, but to its own reputation and once proud shipping brand.
"We are pleased to learn that the government is considering pursuing legal action based on the company’s responses and will also be investigating breaches of the National Minimum Wage and the terms of agency workers’ contracts.”
Transport secretary Grant Shapps has previously indicated that P&O face criminal prosecution with unlimited fines after the firing of staff and the rehiring with £1.82-an-hour agency workers, some living in tents.
Transport secretary Grant Shapps said the lack of consultation has meant that action can be taken in the criminal courts.
Unions have raised concerns over the "exploitative practice" as they told of the low payments, saying they were expected to work huge 12-hour shifts for eight weeks at a time.
The 800 workers were fired with no notice through a view message - with reports some staff were removed from ships in handcuffs.
P&O say that the sacked staff were Jersey-contracted seafarers, while three years ago the company said it was shifting the registration of its UK vessels to Cyprus ahead of Britain’s departure from the European Union. It has raised questions over whether the depth of their legal employment rights in the UK.
P&O, which has received over £50m in UK government contracts and subsidies in two years suspended its services on Thursday before axing the staff.
Services between Cairnryan to Larne, Dover to Calais, Dublin to Liverpool and Hull to Rotterdam were to be out of action for between seven and 10 days.
In a letter Peter Hebblethwaite, current chief executive of P&O Ferries denied "rumours" that security staff who boarded vessels to manage the situation wore balaclavas or were directed to use handcuffs or force.
"The teams accompanying the seafarers off our vessels were totally professional in handling this difficult task," he said.
The company said its settlement with its workers is believed to be "the largest compensation package in the British marine sector," and more than 40 staff would get severance packages of more than £100,000 each.
The transport and freight company said 575 seafarers affected were in discussions to progress with the severance offers.
The RMT union, which has been organising protests over the redundancies said "the pay in lieu of notice is not compensation".
"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," general secretary Mick Lynch said.
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