Labour is urging the UK Government to publish the legal advice it received on whether P&O broke the law when it sacked 800 workers.
The opposition asked whether there are legal moves ministers could take to reverse the decision to make so many workers redundant without consultation.
Labour also published analysis of data it said reveals P&O Ferries has received £38.3m in Government contracts since December 2018.
The party’s leader Sir Keir Starmer called on the UK Government to suspend its contracts with P&O Ferries and its owners DP World over the sacking of its workers until the situation is resolved.
Labour said public procurement and taxpayers’ money should be used to promote good working practices and decent conditions.
It also called for the removal of DP World from the Government’s Transport Advisory Group.
Sir Keir said: “P&O’s actions are aggressive and immoral. The Government’s apparent inaction is as telling about their respect for the security of work as it is damning.
“At the heart of this is a community of workers, some of whom are young and starting out on what they expected to be a long maritime career.
“Many have families, mortgages and responsibilities. Their lives have been upended overnight by an unscrupulous employer and the Government has sat on their hands.
“This Conservative Government bears responsibility. Boris Johnson’s party has created an environment where big companies think they have licence to ride roughshod through good employment practice, basic respect for workers, and the rights of all to be treated fairly.
“If one company can divest itself of responsibility for its workforce, and the Government don’t blink, others have room to do the same.
“We are demanding the Prime Minister grants access to the legal advice the Department for Transport received on whether P&O Ferry’s actions constitute a breach of the law, and whether there are legal levers to get the decision reversed.
“Full transparency is vital to prevent anyone else from being subjected to this appalling behaviour. Any Government contracts they have should be suspended until this is resolved.
“Labour stands firmly with these workers and their union, RMT. It is on all of us to challenge this for workers everywhere who deserve nothing less than security and respect in return for an honest day’s work.”
TUC general secretary Frances O’Grady said: “We have to get to the bottom of this scandal.
“The Government must be transparent and publish the legal advice it has received. That’s the very least the P&O crew deserve.
“If the company has breached the law it must face severe consequences – not just a slap on the wrist.
“What happened at P&O can never ever be allowed to happen again.
“Ministers must urgently bring forward an employment bill to stop workers from being treated like disposable labour. The time for excuses is over.”