Ferry workers are threatening strike action to force Holyrood ministers into protecting their pay and conditions if state-owned ferry services are privatised.

The dispute arises out of union fears that the Scottish Government is preparing to put Caledonian MacBrayne's Ardrossan to Brodick, Wemyss Bay to Rothesay, Oban to Craignure, and Largs to Cumbrae routes out to tender for the new contract from October 2013.

There are concerns this would lead to private companies cherry-picking the busier routes, with different service levels and a knock-on effect on those that could never be commercially viable.

The Union of Rail, Maritime and Transport Workers (RMT) said yesterday that state-owned CalMac had not given assurances that the workers' pensions and working conditions would be protected if there were separate tendering of some services.

RMT general secretary Bob Crow said: "RMT has given CalMac the best part of two months to give us the most basic assurances on pensions, working conditions and bargaining rights, and they have dragged this out to the point where we are left with no option but to ballot for strike action.

"We are confident that our members will deliver a resounding mandate as they fight to protect their rights, their working conditions and their futures."

The Scottish Ferries Review Consultation Document in 2010 had asked whether there was support for market testing of five routes with a view to them being tendered separately.

These also included one on the Pentland Firth from Caithness to Orkney. That was operated by CalMac's sister company, the publicly owned NorthLink Ferries, but has now been taken over by the private sector giant Serco after a separate tendering for Orkney and Shetland.

The RMT has just settled with Serco after threatening industrial action over a threat to working conditions. Now it is looking to CalMac's Clyde and Hebrides services.

A spokesman added: "We're not going to operate with a knife at our throat again. We want this bolted down, and if they say we've got nothing to worry about then why can't they put that in writing."

The union says CalMac has been stalling and promising meetings which never materialised.

A Scottish Government spokesman said: "Claims that we are planning to sell off these routes are simply untrue. The Scottish Government is absolutely committed to supporting our island and remote communities."

CalMac said the decision to ballot workers was regrettable but pledged "to work closely with them to ensure we are able to continue to provide essential lifeline services".