Commuters travelling to Heathrow airport in the coming weeks face several days of delays after train staff voted to strike over pay.
RMT members at Heathrow Express will walk out in a dispute over a three-year pay deal which the union has branded “unacceptable”.
The industrial action will take place on 7, 11, 22 and 25 September.
RMT general secretary Bob Crow said uncertainties about the opening of Heathrow Terminal 5 in two years’ time meant the deal might end up being detrimental to the staff in the long term.
The company’s offer to reduce working hours was rejected by the union, because it would mean the end of a current arrangement which allows reduced-hours shifts to be paid as if a full shift has been worked.
“The company should table an improved offer that our members are prepared to accept,” Crow said.