The ferry operator sacked 800 seafaring staff without warning in a video message last Thursday.
New workers hired by Clyde Marine Recruitment soon arrived at Cairnryan port in Dumfries and Galloway.
But the company insisted it had unwittingly hired replacements - and denied they were cheap labour.
Managing director Ian Livingstone said: "Clyde Marine Recruitment Ltd have supplied crews and officers to P&O for more than 30 years.
"We were as surprised as everyone else in the UK shipping business when the news broke that 600 crew and 200 officers were to lose their jobs with immediate effect.
"We fully understand the anger being felt by the crews, their families and their supporters."
All P&O sailings between Cairnryan and Larne in Country Antrim, Northern Ireland, have been halted since the sackings.
P&O Ferries said the redundancy measures were a "last resort" to save the business.
The Scottish government said it was reviewing all publicly-funded contracts with the operator.
Agency seafarers have told BBC Scotland how they turned and left Cairnryan port when they realised what the job entailed.
Gavin Hamilton, from Paisley, and Mark Canet-Baldwin, from Lincolnshire, said they were given no information about the vessel they would be working on.
The two only realised it was a P&O vessel when their coach pulled up at the dock.
They were accompanied by a dozen security guards with handcuffs.
Mr Hamilton said: "I knew a lot of people on board that were going to be losing their jobs and that just didn't sit right with me.
"When we realised the RMT were involved and this was a big union dispute, we didn't want to be part of that. To us, boarding that ship was like crossing a picket line."
UK Transport Secretary Grant Shapps has questioned whether P&O's actions may have breached employment law.
His Scottish government counterpart Jenny Gilruth has also backed calls for the redundancies to be reversed and said P&O Ferries would undoubtedly suffer reputational damage over its handling of the situation.
The recruitment agency involved is now looking to distance itself from the dispute - and claims that it was providing cheap foreign labour.
Clyde Marine's Ian Livingston added: "The claims that Clyde Marine Recruitment have supplied P&O with foreign crews on lower rates is wrong.
"All the crew and officers we have supplied to P&O are on full UK industry rates."
The RMT union has claimed new crew on P&O ships will be paid at rates well below the minimum wage.
Its general secretary Mick Lynch said: "The weakness in UK employment law has not only allowed the mass dismissing of UK seafarers, it has also incentivised this barbaric behaviour.
"Employers know there may be no effective sanction to stop them doing so, and on top of that they can get away with paying below the minimum wage.
"P&O may pay more than the minimum wage at first to agency staff, but they will eventually move to rates below this simply because there is nothing to stop them from doing so.
"We fear poverty pay will be accompanied by seafarers being chained to 12-hour day, seven-day week contracts that operate continuously for six months, with no pension."
The mass sackings have led to protests at ports used by around the UK.
The RMT has said it will stage a boycott and demonstration at Cairnryan from 12:00 on Wednesday.