Several train companies are suffering problems accepting card payments for tickets.
Among the operators who reported being affected by the issue on Tuesday morning were Greater Anglia, Northern and Southeastern.
Greater Anglia and Northern said the fault meant passengers could not make card payments on their apps or websites, while Southeastern warned that ticket-buying at stations was also affected.
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A spokesman for Greater Anglia (GA) said: “There is an ongoing problem with the GA app and website regarding making card payments.
“This is currently being investigated and will be fixed as soon as possible.”
Northern said in a statement: “We are aware of an issue with our app/website whereby customers cannot complete their booking. This is being looked into and we hope to see a fix soon.
“In the meantime please use ticket-buying facilities at stations to purchase your tickets.”
Southeastern posted on Twitter: “We are aware of an ongoing problem with making card payments at present, at stations and via online channels for ticketing.
“Teams are investigating currently and looking to fix as soon as possible. More details shortly.”