Passengers are facing delays across the UK after engineers found rail workers could not automatically log in to a radio communication system.
National Rail said the fault was discovered in the GSMR radio system, which allows communication between train drivers and signal operators even in tunnels.
A back-up manual log-in system is now in use, but this is causing “a few minutes of delay”.
Latest as nationwide fault hits rush hour
This issue “is currently being investigated”, National Rail said, warning it expects the delays affecting some of its services to last until midday.
“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 added.
Posting an update on social media, it said services across the Gatwick Express, Great Northern, Southern and Thameslink network may be cancelled, delayed by up to 15 minutes or revised.
Services to Heathrow Airport on the Elizabeth line have also 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.
South Western Railway also told passengers its services may be delayed.
For those seeking compensation, National Rail has advised customers to keep hold of train tickets and to make a note of the journey, as both would support any claim.
Helen Dewdney, a consumer rights expert who runs The Complaining Cow site, said passengers are entitled to a 50% refund for a delay of more than 60 minutes.
One passenger travelling from Portsmouth to Basingstoke at 6.32am said he had to pay £54 for a taxi through Uber as it’s the first week of his new job.
Matthew told Sky News he will claim the money back from South Western Railway.
Read more: My train has been delayed or cancelled – can I get a refund?
‘Rare’ fault behind delay
GSMR is designed to enable drivers and signallers to communicate digitally at all times, including while trains are in tunnels or areas where analogue systems do 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.
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The radio fault behind the disruption
Railway journalist Nigel Harris said he hasn’t seen an issue like this in 40 years covering the industry, describing the fault as “very, very rare”.
He suggested it could have come from a technical problem or even an attack by hackers, adding: “We just don’t know.”
“It can take a few seconds to reboot something, or it can cause chaos for a long time,” he told Sky News.
“The tone of the communication from Network Rail seems fairly optimistic, so I would hope that by lunchtime we can get the system back.”