Workers on the Heathrow Express are to stage two walk-outs in a dispute over disciplinary action taken against two employees.
Members of the Rail Maritime and Transport union will walk out for nine hours from 0300 GMT on March 24 and for 12 hours from 1100 GMT on March 25.
The union has already held two strikes in protest at the sacking of a driver and disciplinary action against another worker.
RMT general secretary Bob Crow said: “Instead of using the Acas machinery to negotiate a settlement, Heathrow Express have instead opted to ratchet up their attacks in an act of blatant provocation.
“The unfair and disproportionate punishment taken by management in both these cases is a total miscarriage of justice and that is reflected in overwhelming ballot results for action.
“Management’s behaviour clearly demonstrates their willingness to victimise staff and union representatives at the slightest provocation and no doubt they see this as payback for the successful building of a strong and militant union organisation within the company.”