Cal Mac ratings are to strike at the end of next week.
All main Western Isles ferries and the Sound of Barra service are liable to cancelled on Friday 10 July as crews down tools.
At this stage, it is unclear if the Sound of Harris service will be affected.
Last week's strike caused chaos with no ferries sailing at all.
Tourists and islanders were stranded, businesses lost money, and the subsequent backlog of ferry traffic created more travel problems on the Saturday.
Crews are striking over fears about job losses, cuts to income and pensions in the
face of transferring state-
RMT General Secretary Mick Cash said: “Despite the most strenuous efforts to negotiate
a settlement on CalMac we are still not getting the clear assurances that we seek
that will defend the safety-
"It is a cold hard fact that privatisation is all about a race to the bottom in order to maximise profits and our members will not sit back while they are caught in the crossfire of the tendering process."
He added: "RMT urges the people of Scotland to remain resolute in their support for the workforce in both these disputes – our fight is your fight.”
CalMac Ferries, managing director, Martin Dorchester said:
“We are extremely disappointed that the RMT has chosen to continue with strike action.The best way to protect the existing terms and conditions of its members is to work with us to ensure we win the Clyde and Hebrides Ferry Service contract and this action is damaging.
"We are anxious to avoid any more disruption which is not in the best interests of our employees and the communities we serve and would urge all our colleagues to turn out for work.”
Ferry strike to hit Western Isles next Friday
3 July 2015