Passengers are being warned that a “nationwide fault” is affecting rail services across the network this morning.
National Rail has said the issue is affecting “the communication system used between train drivers and signallers”.
“Trains across the network are having to start their journeys later because of this fault and some may also be subject to cancellations or alterations,” it said.
Passengers are being advised to check before they travel.
Services to Heathrow Airport on the Elizabeth line have been affected, with passengers told to wait for the next available train.
Those using ScotRail are now allowed to travel via “any reasonable route” because of the issue.
The failure is down to a fault with the GSMR radio system, which allows communications between train drivers and signal operators.
This issue “is currently being investigated”, National Rail Enquiries said.
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