The action will hit families heading off at the start of the Western Isles school holidays.

A full strike on Friday is likely to cause severe disruption, Cal Mac has stressed.

Severe disruption to ferry timetables serving 22 islands is expected during an imposed RMT union work- to- rule on Wednesday and Thursday.

Cal Mac cannot guarantee travel to Barra until Sunday due to anticipated travel chaos as seafarers take industrial action.

It also threatens to wreck Barra's hugely popular annual half marathon race and lead to a raft of cancelled hotel bookings at the height of the tourism season.

Some 55% of the RMT’s 585 or so members on Cal Mac ships voted to strike in protest of feared redundancies, cuts and pension changes as the company gears up to fight off private company Serco for the £1 billion tender to run west coast ferry routes.

Cal Mac is working on contingency plans which relies on changing “selected timetables which will be subject to revision during the period of industrial action.”

Ferries with essential supplies may be despatched to outlying islands ahead of industrial action, it is understood.

Alternatively, the firm may reposition vessels to sail as soon as possible in the early hours of Saturday, after the strike ends.

Cal Mac warned passengers: “It has been necessary to stop taking bookings for these dates.

“Should the situation change and industrial action is averted, we will open the booking system back up to our customers.

“CalMac would like to apologise for the impact this disruption may have on our customers.”

Cal Mac said: “The service impact that this industrial action will have is currently being assessed and contingency plans are being compiled at present.”

The shipping firm is phoning around ships and crew members trying to figure out how many are taking part in the dispute.

Navigating and engine room officers belong to a different union and will work as normal.

However, all it needs is for one RMT crew member to down tools as ships cannot legally sail without a full safe manning complement.