Train services between Maidenhead and Twyford are being cancelled and delayed because of signalling problems.
First Great Western said that all lines between the stations were blocked and delays of up to 30 minutes were expected until further notice.
Network Rail engineers are attempting to fix the fault.
UPDATE: 11.45am: Network Rail engineers are at the scene investigating the problem. A spokesman for the firm apologised for any delays caused and said it was trying to fix the issue as soon as possible.
UPDATE: 12pm: Passenger William Moat says he has been sat on a train out of London Paddington for over an hour after it left Maidenhead at 10.54am.
"We are just sat waiting for news from the driver on updates but apparently services in and out of London are affected," he said.
"We left Maidenhead at 10.54am and were supposed to be in Twyford at 11.03am but are still waiting to move."
UPDATE: 1pm: National Rail reporting a reduced service to and from London Paddington until further notice. It says journey times may be extended by up to 60 minutes. The following services will not run: 12 minutes past the hour London Paddington to Reading; 48 minutes past the hour Reading to London Paddington; 00 minutes past the hour London Paddington to Bristol Temple Meads; 15 minutes past the hour London Paddington to Cardiff Central; 55 minutes past the hour Cardiff Central to London Paddington
Heathrow Express and Heathrow Connect services are not affected by this disruption.
UPDATE: 2.45pm: Disruption continuing because of earlier signalling problems. Network Rail engineers have located and fixed the problem near White Waltham, but delays of up to 30 minutes are expected until 5pm.
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