Workers on the Heathrow Express are to be balloted for industrial action in a row over jobs, pay and spending cuts.
Members of the Rail, Maritime and Transport union will vote on whether to launch a campaign of action, including strikes.
The union warned that about 200 jobs were under threat.
Heathrow Express said it was consulting on the changes as it prepares for the arrival of competing service Crossrail.
Acting RMT general secretary Mick Cash told the BBC: “The planned axing of jobs, and the associated freezing and hacking back of pay and working conditions, is a kick in the teeth for the staff who have made Heathrow Express and Heathrow Connect the success story that they are today.
“RMT will mount a massive publicity and political campaign, backed up by industrial action, to stop these outrageous and dangerous cuts dead in their tracks.”
Heathrow Express managing director Keith Greenfield said: “Crossrail will be the first direct competition to us since our launch in 1998 and the message is simple: ‘We must be smarter today to compete tomorrow’.”
He added: “Our proposals will cut our operating costs by nearly £6 million over the next five years, while avoiding the need for compulsory redundancies and protecting existing employees’ futures with the company.”
Crossrail is due to open in 2018 and connect Berkshire and Essex via Heathrow and central London.