A broken rail and shortage of signalling staff is causing major disruption to train services between London and both Heathrow Airport and Reading.
Passenger information website National Rail Enquiries warned Elizabeth Line, Great Western Railway and Heathrow Express services are affected.
Trains are unable to run between Heathrow Airport and Paddington, and services between Paddington and Reading are subject to cancellation or delay.
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Disruption is expected to continue until the end of Tuesday.
Network Rail engineers discovered the broken rail on the high-speed line towards London Paddington from Slough/Heathrow.
The defect means the speed limit for trains has been reduced, causing delays.
Meanwhile, a member of signalling staff has been taken ill.
A replacement was due to arrive “on site” at around 8.30am, National Rail Enquiries said.