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."