HMRC is believed to be assessing whether P&O’s alleged failure to adhere to minimum wage laws would allow the UK to block them from British ports.

The tax agency said it could not comment on individual cases. But a spokesman added: “HMRC won’t hesitate to take action if the law is broken to ensure that workers receive what they are legally entitled to. Paying the national minimum wage and national living wage is not optional – it is the law.”

P&O is looking increasingly less likely to face criminal charges and an unlimited fine after using a law signed off by former minister Chris Grayling to its advantage.

Mr Grayling, the former transport secretary, signed off legislation that meant maritime companies do not need to notify the Government about mass redundancies if their vessels are registered overseas.

Mr Hebblethwaite told Kwasi Kwarteng, the Business Secretary, on Tuesday: “All relevant vessels are registered outside of the UK.”

Andrea London, an employment lawyer at Winckworth Sherwood, said: “There is a requirement to pre-notify the “competent authority” of the country of registration, but not if the crew [ordinarily] work outside Great Britain.

“As for the unfair dismissal claims, the employees would have to show they had a “sufficiently strong” connection to the UK for these to come within UK jurisdiction – which if the ships are foreign registered and the employing entity is foreign registered and those ships rarely in the UK, so the staff work ordinarily outside of the UK – this might well cause jurisdictional issues.”

Mr Hebblethwaite is set to make his first public appearance on Thursday following last week’s sackings. He will be grilled by MPs from the House of Commons business and transport committees.

Issuing his apology on Wednesday he defended the need to sack seafarers and replace them with agency staff.