Rail travellers to Heathrow from London Paddington face disruption today on the busiest pre-Christmas travel day.
Heathrow Express and Heathrow Connect services were suspended this morning while First Great Western departures to Reading and the west country face delays of up to an hour.
National Rail issued the warning as a result of a signalling problem near Hayes and Harlington.
Delays are expected until around 3pm this afternoon.
Heathrow Express said: “Due to ongoing unplanned engineering work by Network Rail in the Acton and Stockley areas all Heathrow Express services are currently suspended.
“We apologise for any disruption that this may cause to your journey.”