The Yell Sound ferry Daggri will be taken out of service for up to a fortnight in early July in order for essential mechanical work to be done.
According to Shetland Islands Council, inspections during the recent annual refit had highlighted an issue with the shaft bearings between each main engine and the propulsion thrusters.
Parts were not available at that time and the vessel would now need to be taken out of service to carry out the repairs, which could take up to two weeks.
The Daggri will be taken off the route after the last scheduled sailing on Friday 5th July and replaced by the much smaller Fivla, which will operate alongside the Dagalien.
Bookings and timetable on the route would operate as normal but with a reduced capacity for vehicles, the SIC said, apologising for any inconvenience caused.