No vehicles will be carried on the ferry between Whalsay and the Mainland from Saturday evening until early on Monday morning.
The SIC said "essential linkspan maintenance" meant there would be no ro-ro service to or from Whalsay after the 6.30pm service from Laxo and the 7pm service from Symbister on Saturday evening. Normal service will be resumed at 6.30am on Monday.
The passenger service will continue to operate with an altered timetable on Sunday, but from the old breakwater rather than the normal berth.
In a statement, the ferries department apologised for any inconvenience caused. Up-to-date information is available by calling (01595) 743975 or (01806) 244225.