The Island's Trade Union Council has raised concerns about driver fatigue during the strike, now in its sixth day.
Bus Vannin's limited services have been run by a small number of drivers on a contingency timetable since industrial action began last week.
The union organisation's pointed out services are operating from 7am until just after 8pm today, raising concerns about excessive driving time for staff.
It adds managers are also being re-deployed to cover drivers, and questions who'll be overseeing safety and operations as they do so.
Under current legislation, there are already strict rules about the hours which drivers can be on the road for.
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