Tonight’s sailing of the NorthLink ferry Hjaltland from Aberdeen to Lerwick will not depart until midnight.
The company said forecasted bad weather was responsible for the five-hour delay, with the ship having been due to leave at 7pm.
The Hjaltland is expected to arrive at Lerwick at around noon tomorrow.
The delay was announced shortly after NHS Shetland made the decision that in future most patients travelling to hospital appointments would have to do so by ferry.
Both cargo ships, the Helliar and Hildasay, would also see their services disrupted over the next 24 hours, NorthLink added.