Heathrow Express bosses are threatening to sack staff to bankroll losses caused by strikes.
In a statement, Heathrow Express said: "Job losses cannot be ruled out at Heathrow Express if the RMT union presses ahead with further wrong-headed strike action."
Heathrow Airport is experiencing disruption of flights both in and out of it due to fog.
Passengers are advised to check the status of their flight with their airline. Flights may be delayed or cancelled.
There is also disruption to flights at London City Airport due to low visibility.
Passengers advised to contact their airline for more information.
A Heathrow Express spokesperson, said:
There will be no Heathrow Express services until 2.30pm this afternoon due to a strike by rail workers. It's in protest over the sacking of a driver.
Heathrow Express staff have called a 24-hour strike in protest at disciplinary action against two colleagues. They are walking out from midday today, hitting services on the rail link between the airport and central London.