The dispute concerns maritime pilots who navigate ships - including cruise ships - in and out of the UK’s waterways and ports.

Vessels cannot leave or enter UK’s ports without these workers, who have expert knowledge of the weather, tides, water depths and traffic management. 

Unite said ABP introduced increased medical standards at ports without any consultation last July. 

The trade union added that a consultation is required under health and safety legislation and its recognition agreement with ABP.

An ABP spokesperson said: “ABP is continuing to engage with our recognised trade unions and we are hopeful of reaching a resolution. 

“We are pleased that the unions acknowledge the benefits of the new health assessments for our marine colleagues, which we continue to believe are very much in the best interests of their members."

The spokesperson added: “ABP has not been formally notified of an intent to move to industrial action.” 

The majority of Unite members are at ports in south Wales, Southampton, and the Port of Hull, however the dispute could impact all 21 ports operated by ABP.

Unite said: “Unite has no objection to improving standards, but there has been no negotiations and no detail about how these medical tests will be done or what happens if a member fails.”  

Unite general secretary Sharon Graham said: “Maritime pilots are scarce, skilled and highly experienced. Ships can’t leave or enter the UK’s ports without them. 

“So, it’s all the more incredible that ABP Ports is refusing to negotiate important changes to their health and safety. 

“Unite is not opposed to enhanced checks, but they need to be negotiated and introduced fairly.”