Ferry workers are to be balloted on possible industrial action in a row over pensions.
The RMT union is concerned that conditions will change when Serco takes over the operation of ferry routes in the Northern Isles from publicly owned NorthLink next month.
The union claims that the change will have an adverse impact on pensions and lead to possible job losses.
RMT General Secretary Bob Crow said: "RMT always said that this deal was about maximising profits at the expense of services, fares and the workforce and our fears are proved well-founded by this pensions scandal."
Serco will take over as the operator of the Northern Isles Ferry Service from July 5.
Jamie Ross, Business Development Director at Serco, said: "We have had a series of recent meetings with staff and unions and the discussions have been constructive.
"Over the coming weeks we will continue working with both the unions and employees to discuss a number of issues."