An outbreak of sickness and diarrhoea has forced NorthLink to take one of its ferries off the Aberdeen to Shetland route.
The Hjaltland has been withdrawn from service after sickness affected a bus tour party from Hereford who had travelled from Lerwick to Aberdeen.
The company's sister vessel Hrossey will run a continuous shuttle service between Lerwick and Aberdeen.
NorthLink expects to resume normal service on Thursday.
Three crew also later became ill.
NorthLink chief executive Bill Davidson said: "The vessel is being taken out of service as a precautionary measure.
"The master of Hjaltland immediately implemented the company's cleaning and hygiene regime designed for outbreaks of this nature.
"Passengers are being informed of the new arrangements and to them we apologise."