The Department for Transport canceled a contract with P&O Ferries after it conducted a review of government business with the firm, after the sackings back in March 2022.
The Ministry of Defence revealed it had used P&O "to support a recent exercise."
In March, P&O Ferries replaced its sacked staff with foreign agency workers who were paid less than the minimum wage.
The company's ferry services were suspended after the mass sackings and a number of ships failed safety inspections prior to being cleared to restart operations.
Grant Shapps (Transport Secretary) accused P&O of "acting like pirates of the high sea" and initiated a review of the UK government's contracts which led to the termination of a contract between P&O and the Home Office.
According to the UK's Department for Transport, P&O was not providing any direct services to the UK government. It later said it couldn't rule out the possibility government departments would use the ferry services on an ad-hoc basis.
Now it has emerged that the MoD had used P&O while air transport had been busy supporting Ukraine.
Responding to the military use of P&O vessels, a government spokesperson said they didn't have any contracts with P&O.
"The Ministry of Defence occasionally require specific logistics operations to support national and international security arrangements.
"P&O Ferries are the provider of last resort in such situations, on an exceptional basis only,"