Rail staff could be forced to take industrial action if pensions are threatened after firms were barred from bidding for franchises, a union has warned.

Last week, the Department for Transport (DfT) banned Stagecoach from bidding for train franchises after it refused to take on the full historic pension liabilities of railway workers.

Virgin boss Sir Richard Branson warned that his train business, which Stagecoach has a 49% ownership stake in, could disappear from the UK as a result of the move.

At the Scottish Trades Union Congress (STUC) conference in Dundee on Monday, Mick Cash, general secretary of the Rail, Maritime and Transport (RMT) union, is expected to say that the situation “lays bare” the problems around privatisation of the railways across the UK.

Mr Cash will say: “The fiasco over Stagecoach and Virgin being banned from the franchise lottery because they refuse to underpin ‎their pension obligations has laid bare the chaos of privatisation and has left RMT members, including many in Scotland, facing a period of deep uncertainty.

“That scandalous situation has to be brought to an end as a matter of urgency.

“RMT has made it clear that any threat to our members’ pensions, jobs or working conditions as a result of a politically driven privatisation crisis they are not responsible for will be met with co-ordinated and robust action.”

In his speech to conference, Mr Cash is also expected to reiterate calls for the re-nationalisation of the railways in Scotland.

He will say: “In Scotland, there is no excuse for allowing the privatisation racket to continue any longer. ScotRail should be taken into public ownership and run as a public service in the interests of safe, secure and reliable services for all.

“The Scottish political establishment need to face up to both that reality and their own obligations and take the decisive action required.”

Scottish Greens transport spokesman John Finnie MSP said: “The privatisation of our railways has been an unmitigated disaster.

“The Scottish Government should stop prevaricating and take steps to strip the abysmal Abellio off the ScotRail franchise, putting in place a public sector operator as soon as possible.”

A Transport Scotland spokeswoman said: “Franchises in other parts of the UK are a matter entirely for the Department of Transport and the bidders concerned.

“Calls to rip-up the contract now fail to acknowledge the issues which would arise, not least the expense this would bring to tax-payers coupled with the disruptive impact on passengers and staff alike.

“They also ignore the fact another contract would have to be let under the existing system and yet this would do nothing to address the current issues impacting performance.

“Ministers have repeatedly stated that franchising, which is reserved to the UK-Government, no longer works in its current format and that is why they continue to push Scotland’s interests, including further devolution of rail powers, to the UK-wide Williams review.

“They have also demonstrated their commitment to railway employees by including pensions, the Living Wage and no compulsory redundancies as integral parts of the existing ScotRail and the Caledonian Sleeper franchise contracts.”