The west coast ferry operator has already been forced to cut sailings on ten of its 26 routes because of the rate of staff absence, including suspension of the Mallaig-Lochboisdale link.
The reduced “essential services” timetable, introduced last week, is due to continue until at least Sunday, January 23.
CalMac said more than twice as many front line staff were absent after testing positive, self-isolating or waiting for test results than when the timetable was announced on December 31.
It said the absences were placing “immense pressure” on operations.
CalMac said 151 crew and 35 port staff were unavailable because of Covid – or 20.3 per cent of all staff.
That compares to 93 crew and 18 port staff on December 31, and 35 crew a week before.
A further 6 per cent of staff were absent for other reasons, which is unchanged on two weeks ago.
The operator said the temporary timetable was “helping to focus resources on maintaining lifeline routes, but disruption is likely to continue for some time”.
It said maritime law required a minimum number of crew on board.
Sailings might have to be cancelled if crew members tested positive for Covid and replacement crew could not be found in time.
CalMac managing director Robbie Drummond said: “The absence of so much of our frontline staff is placing immense pressure on our ability to maintain a normal service.
"We are now focusing as much of our resources as possible on keeping essential deliveries going to our communities.
“There is a possibility that your sailing may be disrupted or cancelled, so please check the status before you start your journey to a port.
"As this situation is constantly developing, cancellations may also take place at very short notice.
“Help to protect passengers and crew by wearing a face covering in port offices and inside ferries, and please do not travel if you have tested positive and are still within the self-isolation period, even if you feel better.”
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