Workers on South Western Railway will engage in 27 days of strike action over Christmas in a long-running dispute over the role of train guards.
The action is set to take place between December 2 and New Year's Day, threatening travel chaos over the festive period.
The Rail, Maritime and Transport union (RMT) has said South Western Railway had failed to confirm that they would avoid moving to Driver Controlled Operation, where the driver is in control of the opening and closing of a train's doors.
The union argued that this move would leave the role of the guard “butchered completely”.
RMT general secretary Mick Cash said member had been left with “no choice” but to take industrial action after feeling “hugely let down” by negotiations.
The union said the company had “dangled a potential breakthrough deal” in front of members but then “failed to honour it”.
A South Western Railway spokesman said the company is “extremely disappointed” at the planned action, accusing the union of a “typical of the lack of concern” for customers.
The company had reportedly offered to keep a guard on every train if the RMT works with it to agree an operation method for new trains which “fully utilises the new technology".
The spokesman continued: “Instead, the RMT appear purely focused on keeping control of train doors in a misguided attempt to hold power over the industry.”
Boris Johnson's official spokesman urged the warring groups to solve the issue and remove the threat of the “deeply damaging” action.
The strike announcement comes as other major railways are also set to be disrupted over Christmas due to engineering work on the Crossrail project.
Great Western Railway, Heathrow Express and TfL Rail will be affected, with London Paddington closing from December 24-27 and upgrade works also taking place at London King’s Cross.