Workers on the Heathrow Express will stage another strike on Sunday in protest at disciplinary action over two rail workers.
Members of the Rail Maritime and Transport union will walk out for 24 hours from just after mid-day, hitting services on the rail link between the airport and central London.
The workers also went on strike last month over the sacking of a driver and the suspension of a union rep after voting for action in a ballot.
RMT General Secretary Bob Crow said: "Instead of using the period between the ballot result and the announcement of industrial action 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 the overwhelming ballot results for action.