The government has admitted P&O Ferries was used by the military - despite widely condemning the firm for sacking nearly 800 staff without notice.
The Department for Transport cancelled a contract with P&O after it conducted a review of government business with P&O, after the sackings in March.
The Ministry of Defence said it used P&O to support a recent exercise.
It came after the RMT transport union said it saw evidence the MoD had bought slots on P&O's Dover-Calais service.
In March, P&O replaced its sacked staff with foreign agency workers paid less than the minimum wage.
Its services were suspended after the mass sackings and several of the company's vessels failed safety inspections before being cleared to resume operating.
The reduced ferry capacity was one factor in long lorry queues building up around Dover before Easter.
Transport Secretary Grant Shapps accused P&O at the time of "acting like pirates of the high sea" and initiated a review of the government's contracts - which led to the termination of a contract between the Home Office and P&O Ferries.
The Department for Transport said that P&O was not providing any direct services to the government. It later said it could not rule out the possibility government departments would use P&O services on an ad-hoc basis.
It has now emerged that the Ministry of Defence used P&O Ferries while air transport was busy supporting Ukraine.
Responding to the military's use of P&O, a government spokesperson said: "We do not 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," the spokesperson added.
RMT general secretary Mick Lynch described the MoD's use of P&O services as "new low, even by this zombie government's sinkhole standards".
He urged the government to issue a statement prohibiting public contracts with P&O Ferries and its parent company DP World.