Workers on the Heathrow Express are to stage a fresh wave of strikes in a continuing dispute over disciplinary action against two rail workers.
Members of the Rail Maritime and Transport union will walk out for nine hours from 3am on Saturday March 24 and for 12 hours from 11am the following day, disrupting services between the airport and central London all weekend.
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."
A Heathrow spokesman said: "Heathrow Express and the RMT are currently in discussions with the assistance of Acas.
"We are disappointed in the union's decision to take industrial action on the 24th and 25th March, even while Acas talks are in progress to resolve the dispute constructively.
"The dispute centres around the suspension of one member of staff and the dismissal of another.
"The suspended employee has already returned to work and the dismissed employee has withdrawn his employment tribunal claim for unfair dismissal.
"We hope union members will recognise that industrial action is unnecessary and we will continue to talk with the union to try and resolve the dispute."