Three days of industrial action by CalMac ferry workers has begun, with union chiefs insisting the protest is about defending lifeline services to Scotland's islands.

Ferry staff in the RMT union will work-to-rule today and tomorrow, before taking all out strike action on Friday.

The dispute has been sparked by union fears that a tendering bid could see routes change hands from publicly-owned CalMac to private firm Serco.

As the industrial action got under way today, RMT general secretary Mick Cash said the protest was "about defending the lifeline ferry services long into the future".

He said: "The whole history of privatisation is well documented and means inevitable cuts to staffing, attacks on pay, pensions and conditions, and the cherry-picking of lucrative services to maximise profits. Public service and loyalty to staff doesn't get a look in.

"CalMac has a great history. RMT wants it to enjoy a great and secure future as well. That means respecting the staff who are out there round the clock, often in appalling conditions, delivering these lifeline services."

The ferry firm has already said it expects the majority of its services to run today and tomorrow. But on Friday, major vessel routes serving the Western Isles, Mull, Islay, Bute, Arran, Coll and Tiree are likely to be severely affected and only supported by smaller vessels in most cases.

Mr Cash insisted: "Let me be clear from the outset: our dispute is not with the users of the ferry services - it is with the company."

He said it is "unfortunate that individual plans will be disrupted", but added: "This dispute was wholly avoidable if those calling the shots had engaged with the union and listened to us from the outset. They have chosen not to.

"RMT's action is about defending the lifeline ferry services long into the future. Our members have been forced to take action due to the failure of the company to respond, despite ample warning, to the union's calls for assurances that would protect both the livelihoods of the loyal and hard-working CalMac staff and the essential ferry services that they provide.

"Our members are active and respected members of the communities that rely on these CalMac services. The idea that they would take action lightly is ridiculous. Many of them rely on the ferry services themselves, not just for their jobs but for their own transportation and supply lines.

"The last thing that they would want is to inconvenience their own friends and neighbours, but when you have had your arm twisted up your back, and you know exactly what is looming on the horizon, there comes a point when you either make a stand or you roll over.

"Our members have chosen to make a stand in ballots showing over 90% in favour of action - our trade union is proud of them."

He called on the public to "stand by the workforce and help us to secure a fair settlement".

CalMac has said it has made "several serious concessions to the RMT including a commitment on no compulsory redundancies", which have "unfortunately" been rejected by the union.