Heathrow Express trains have been suspended this morning due to severe damage to overhead electric cables on the line.
Network Rail said all Great Western Railway trains between Paddington and Slough have been cancelled until at least midday and would likely be affected all day.
A bus replacement service has been set up to take passengers from London Paddington to the airport.
The damage to the cables near Ealing Broadway last night left many trains stranded on the line between Paddington and Reading.
"Network Rail are working to recover all the stranded trains and to rectify the damage to the overhead electric wires. Network Rail expect to reopen two of the four lines from midday. However trains will remain disrupted until the end of the day," it said.
