The company was widely criticised for making the seafarers redundant without notice on March 17.
Business Secretary Kwasi Kwarteng said the Insolvency Service has started “formal criminal and civil investigations”.
The Insolvency Service said: “Following its inquiries, the Insolvency Service has commenced formal criminal and civil investigations into the circumstances surrounding the recent redundancies made by P&O Ferries.
“As these are ongoing investigations, no further comment or information can be provided at this time.”
P&O Ferries chief executive Peter Hebblethwaite told a joint hearing of the Commons’ business and transport committees that his company broke the law by not consulting with trade unions before sacking workers.
Responding to the launch of the Insolvency Service’s investigations, Transport Secretary Grant Shapps wrote on Twitter: “Peter Hebblethwaite stood before MPs and admitted to breaking the law, and his actions must now be scrutinised.”
Mr Hebblethwaite insisted the company would not have survived without taking the action it did.
He told MPs the average pay of the agency crew is £5.50 per hour.
That is below the UK’s minimum wage but Mr Hebblethwaite said this is permitted under international maritime laws.
On Wednesday, Mr Shapps unveiled a package of measures in response to the sackings.
They included plans to create “minimum wage corridors” on ferry routes between the UK and other countries.
He also urged UK ports to refuse access to boats carrying seafarers paid below the minimum wage, and asked the Insolvency Service to consider disqualifying Mr Hebblethwaite from acting as a company director.