Heathrow Express passengers will face disruption over the Christmas period due to planned engineering works.
The service, which starts at London Paddington station, will be suspended for six days because of work on Crossrail.
Rail bosses recommend people plan their journeys using the National Rail Enquiries website.
"There's never a good time to impact on journeys and I'd like to thank passengers in advance for their patience,” said National Rail's England and Wales managing director Phil Hufton.