The UĦM has ordered sailors employed by contractors on board Gozo ferries not to clean toilets or berth the vessels. They were also told not to go down to the garage.
The action was taken to press for better pay for the workers compared to their colleagues doing the same work and employed by the ferry company.
The directives come into force on Monday.
The UĦM said the workers were demoralised by their situation. Talks with the Director of Employment Relations on Monday last week were inconclusive.