Staff on the Heathrow Express are due to take strike action next week in a row over pay.
The RMT union says members will walk out for 48 hours from Monday in protest at a “below inflation” offer.
The firm, which operates direct shuttle trains between Paddington and Heathrow Airport, described the proposed walkout as “incredibly disappointing” but said services would be unaffected.
A Heathrow Express spokeswoman said: “It is incredibly disappointing that the RMT is planning strikes, but our well-planned contingencies are ready to go.
“Schedules will continue as normal, and we will keep our passengers moving safely and efficiently.
“The strike… does not include our train drivers and schedules will continue as normal. There will be no disruption to Heathrow Express services as a result of this action.”
According to the RMT, the offer of a 3.5 per cent increase was below the RPI rate in January, the month used for pay calculations.
The offer was “overwhelmingly” rejected by RMT members, the union said. It said it only called the strike after “multiple” attempts to resolve the dispute.
The staff taking action work as sales and service ambassadors.
RMT General Secretary Mick Lynch said: "Our members at Heathrow Express have made their position clear with a strong mandate for action. They are determined to secure fair pay and better working conditions.
"Heathrow Express management must now recognise the serious concerns of the workforce and return to the table with a meaningful offer."
The union said it remained open to further negotiations, and called on Heathrow Express management to “improve their offer and avoid ongoing disruption”.
Heathrow Express trains are operated by Great Western Railways. It is the busiest “open access” rail operator in the country, carrying 16,000 passengers a day.
Open access operators do not run services under Government contracts and are responsible for all profits and losses they make. In June, the Heathrow Express extended its track access contract until 2028.
Heathrow Express offers a “premium” service but has faced stiff competition from the Elizabeth line, which offers cheaper standard fares.
