The ferry operator says that while the immediate future of services on the Clyde and to the Outer Hebrides is secure, some changes may have to be made to conserve fuel supplies if the strike by workers at Grangemouth drags on.

These could include running ferries at a slower speed or reducing service frequency to the level of the winter timetable - though the company has promised that no island would be left without a service entirely.

CalMac managing director Lawrie Sinclair said: "We will endeavour to keep our customers and stakeholders fully informed of the current position and any impact changes may have on services.

"There is currently no need for customers or staff to change existing travel or work patterns although staff are being advised to be as economical as possible. A robust temporary refunds policy will cover any period of service disruption which may take effect in the event of the supply situation becoming a problem.

"I would, however, encourage customers not to cancel reservations in the interim, as we are confident that we will be able to maintain continuous services for some time, albeit some of them may be at a reduced level to help maximise our stocks."

Some of CalMac's larger vessels, such as MVs Hebrides and Clansman, use a different grade of fuel which is not supplied from Grangemouth and thus should be largely immune from the direct effects of any shortages.

The company's operations director Phil Preston said: "We will maintain a full service as long as possible and that industrial action will not impact on our operation; we are taking measures to secure fuel supplies.

"If action is taken, we will put in place contingency measures to prepare for a range of possible scenarios. We are confident that we have sufficient stocks in hand, for us to be able to ensure that the published services will be maintained to all islands going forward.

"Obviously, if the situation is to get worse, some service alterations will have to take place to ensure that we can maximise the extent of the stock available and this could take the form of slowing ships down, or reducing services to some islands, for example to winter timetable level.

"In the meantime, customers should be assured that no communities will be left without a reasonable level of lifeline services."

CalMac has also warned its passengers that it will not allow spare cans of fuel to be carried on board its ships, and dangerous goods must be declared at least 24 hours in advance to the port of departure.