The RMT union has warned P&O Ferries that it faces a "summer of disruption" when it resumes passenger services from Scotland to Northern Ireland this week.
P&O sparked outrage when it sacked 800 staff last month and sought to replace them with cheaper agency workers.
Passenger services from Cairnryan to Larne have been suspended ever since, but are now due to resume on Thursday.
RMT regional organiser Gordon Martin told BBC Scotland: "The RMT and our comrades in the trade union movement and beyond will be at Cairnryan and other ports on a regular basis, blockading the ports - nothing in, nothing out."
He called the company "capitalist bandits" for the manner in which it sacked staff via video call with no prior notice - and without informing unions.
"We will be after you, we will be coming to Cairnryan, Larne, and elsewhere and we will be targeting your supply chain.
"Anybody dealing with P&O - I'll send a clear message to them - whether it be fuel providers, food providers, whatever it may be," Martin continued, adding: "We will find out who you are and you will be part of our efforts to get proper legislation in place to protect workers."
Following the mass sacking, P&O's European Causeway ferry - which operates between Cairnryan and Larne - was detained in the Northern Irish port over safety concerns.
The Maritime and Coastguard Agency raised concerns about "failures on crew familiarisation, vessel documentation and crew training".
On Saturday, it began sailing again with limited services. It is understood that only freight was transported on the vessel.
On Sunday, P&O announced on Twitter that the route would resume for tourist travel on 14 April.
Stena Line have put on extra ferries from Cairnryan to Belfast to compensate for the postponement of P&O services.
The Scottish Government condemned the "serious disruption" P&O's actions had caused to the supply chain between Scotland and Northern Ireland.
A spokesman said: "The employment practices P&O have needlessly pursued have no place in a modern working practices.
"The Scottish Government has been absolutely clear that we will stand by the workers who have been let down in the most appalling manner imaginable by P&O."
Meanwhile, the UK Department for Business, Energy and Industrial Strategy stated: "The Insolvency Service has confirmed it has commenced formal criminal and civil investigations into the circumstances surrounding the recent appalling behaviour of P&O Ferries, following the Business Secretary asking they undertake a thorough review.
"We will not provide further comment while investigations are ongoing."
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