A new railway signalling system being trialled in the UK could be vulnerable to a cyber attack which could cause a train crash, a government adviser has warned.
Professor David Stupples told the BBC the European Rail Traffic Management System, a new digital system aimed to make lines safer, could be exposed to malicious software, or malware, used to cause a "nasty accident".
The internet security expert at City University said government ministers feared the possible threat of hacking and the biggest could come from an insider.
"It's the clever malware that actually alters the way the train will respond. So, it will perhaps tell the system the train is slowing down, when it's speeding up," he said.
The professor added that safeguards were being introduced but were not foolproof.
"The weakness is getting malware into the system by employees. Either because they are dissatisfied or being bribed or coerced."
Network Rail, in charge of the upgrade which is expected be operational by the 2020s, said it acknowledged the threat.
"We know that the risk (of a cyber attack) will increase as we continue to roll out digital technology across the network," a spokesman said.