Train drivers on the Gatwick Express are to go ahead with a one-day strike on Friday after talks broke down in a dispute over pay.
Thousands of passengers face disrupted journeys as members of the train drivers' union Aslef walk out for 24 hours.
Further stoppages are planned for Christmas Eve and New Year's Eve.
Gatwick Express said it would run a "comprehensive" service on strike days, although the number of trains is expected to be reduced by half.
The company, owned by National Express, said it had offered its drivers an above inflation pay rise of 7.5% over two years.
Sunday working
The 60 or so drivers on the line, which operates between Gatwick Airport and London's Victoria station, want a 6% pay rise this year rather than a rise spread over two years.
But the union said the talks broke down over the issue of payments for Sunday working rather than the exact percentage pay rise.
Management said the deal had been an improvement on what had been offered during the past five months of talks.
Managing director David Stretch said he was disappointed by the industrial action, particularly the proposed Christmas Eve and New Year dates.
He said: "We shall continue to operate a comprehensive service throughout the industrial action to minimise the disruption and inconvenience caused to our passengers by Aslef's decision to strike."