The Trade Unions Congress (TUC) has called for a public and commercial boycott of P&O Ferries.
Government ties with its Dubai-based owner DP World should also be severed, according to the union organisation.
P&O Ferries sacked nearly 800 seafarers with no notice last month, replacing them with cheaper agency workers.
TUC general secretary Frances O’Grady said: “P&O and DP World must not be allowed to get away with their scandalous and unlawful treatment of staff.
“Companies who behave like corporate gangsters deserve far more than a slap on the wrist.
“Ministers must sever all commercial ties with P&O and its owner DP World and ensure they do not receive a single penny of taxpayers’ money.
“After their appalling behaviour P&O deserve to be given pariah status.”
The TUC said DP World has “lucrative Government contracts” to run the Port of Southampton and London Gateway port in Thurrock, Essex.
The PA news agency understands the company expects to restart sailings between Dover and Calais for freight customers on the Spirit of Britain vessel by Wednesday.
Passenger services are expected to resume early next week.
P&O Ferries has not operated between Dover and Calais since the mass sackings on March 17.
A spokesperson for P&O Ferries said: “We’re pleased to have welcomed tourist passengers and freight customers back on board some of our ships and we’re looking forward to having all of our services running again as soon as mandatory safety tests have been passed.
“P&O Ferries is now a modern, dynamic, competitive and viable business which can meet customer needs flexibly in a way that has not been possible in the past.
“As part of our long term commitment to the business, we are investing in a new generation of fully efficient, low impact ships on Dover-Calais.
“In addition, a new class of ship – due to enter service in 2023 – will reduce turnaround time in the ports, reducing operating costs and emissions.
“Our newly formed and forward looking operational relationship with DFDS is also focused on customer service through delivering greater flexibility, more frequency and easier travel.
“As we have also already made clear, we will continue to comply fully with any national minimum wage obligations introduced by the UK Government.”
Spirit Of Britain was detained by the Maritime and Coastguard Agency (MCA) on April 12 after safety issues were found, but was cleared to sail on Friday.