MV Isle of Lewis will have to return to Liverpool for further repairs.
The vessel only returned to service at the weekend after a refit which had taken longer than expected. She experienced technical problems on Saturday and MV Isle of Arran was sent to Stornoway to cover the route.
A CalMac spokesman said: “The MV Isle of Lewis has a rudder fault which will require her to return to dry dock for repair. We had hoped that it could be corrected while berthed in Ullapool, but specialist engineers advise this is not possible. Unfortunately, the only suitable dry dock facilities available are in Liverpool, so she will leave Ullapool once we are satisfied she can sail there safely. The cause of the rudder issue is not known, and more detailed investigations will be carried out when she is in dry dock. It is too early to say when repairs will be completed and she can return to the route, but we will do everything we can to minimise her time out of service. In the meantime, MV Isle of Arran will maintain the regular service. CalMac apologises for any inconvenience caused.”