Talks over averting this week's ferry strike have broken down.
The RMT union confirmed that industrial action affecting Western Isles ferries will take place.
It means travel chaos is anticipated at the very start of the Western Isles school holidays.
Barra’s half marathon race will also be hit with the possible consequence of cancelled hotel bookings.
The spectre of private firm Serco winning a £1 billion subsidy over eight years and snatching the lifeline sea routes to 22 islands is at the root of the row.
Cal Mac insists changes are required to “safeguard the pension fund by ensuring it is affordable and sustainable for years to come, while also offering value to the Scottish taxpayer.”
Seafarers in the RMT trade union imposing an overtime ban on Wednesday and Thursday which would affect any ferry delayed by breakdowns or volume of traffic.
Effectively, any ferry not operating to timetable could be hit.
On Friday, many ships are liable to be stuck in port all day as deck, engine room and catering ratings down tools.
However, striking crews will work if the safety of the ship requires it at any time.
RMT General Secretary Mick Cash said: "The talks with CalMac have not made significant progress despite the best efforts of our negotiators and as a result the action goes ahead from Wednesday.
"It is extraordinary that rather than standing up for Scotland's lifeline ferry services those who hold political power have resorted to ludicrous arguments about what does and doesn't represent privatisation.
"That is a kick in the teeth for loyal and hard working staff who have given long hard years of service at CalMac."
He added: "RMT members on CalMac rightly feel that they are caught in the crossfire of an unnecessary and damaging tendering battle that leaves jobs, conditions and pensions hanging by a thread. That is simply intolerable.
"RMT wants cast iron assurances and we want them now and we are committed to engaging in meaningful talks around that agenda as we kick off the first phase of industrial action."
Cal Mac ferry strike is confirmed, talks break down
22 June 2015