Train passengers are being warned they could face delays and disruption this morning.

A “nationwide fault” is affecting rail services across the network, National Rail has said.

It said the issue is being investigated.

A problem with a communication system used by train drivers and signallers is causing services to start journeys late and could cause disruption on Friday morning, the group said.

National Rail said: “There is a nationwide fault with the communication system used between train drivers and signallers.

“As a result, services across the National Rail network may be subject to disruption this morning.

“Trains are disrupted on various routes because of a nationwide fault with the onboard "GSMR" radio system that allows communication between train drivers and signal operators. This issue is currently being investigated.

“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.

“Please check before you travel, allow extra time for your journey and monitor live departure boards.”

South Western Railway is among the operators to cancel some services.

It said in a message to passengers: “We have been informed of a national issue with communication systems affecting service this morning.

“This means that trains may be delayed before they start their journey.

“This is because it is preventing our train drivers from contacting the respective signalling centre this morning and a reset is having to be performed before the train can start its journey.”

Elizabeth line trains serving Reading and Heathrow airport are affected.

ScotRail passengers are being permitted to travel via “any reasonable route” because of the issue.

Gatwick Express, Great Northern, Southern, Thameslink and Southeastern services are also disrupted.

The failure with the GSM-R system “is currently being investigated”, National Rail Enquiries said.

GSM-R is designed to enable driver and signallers to communicate digitally at all times, including while trains are in areas such as tunnels and deep cuttings where previous analogue systems did not work.

It was rolled out to increase safety and reduce costs by replacing the patchwork of inefficient legacy systems which were expensive to maintain.