The UK Maritime and Coastguard Agency (MCA) have detained 2 P&O ferries after they were inspected and deemed unfit to sail.
The news comes after P&O sacked 800 members of its shipping staff and replaced them with agency staff earning less than minimum wage.
P&O have already faced extensive criticism for the move, and are now facing accusations that getting rid of experienced staff is putting customer safety in danger.
The UK Transport secretary Grant Shapps has announced plans to force ferry operators in the UK to pay all staff the national minimum wage. They can currently dodge minimum wage rules if their ships are flagged outside the UK.
Shapps has also proposed that ferry operators breaking the rules should be blocked from ports. Port operators have said this plan is unfeasible and creates other legal issues, however.
P&O's ferries carry both passengers and ro-ro freight. If more vessels are taken out of service road freight movements to and from mainland Europe and the UK could face serious disruption.