Failures to properly maintain a railway tunnel could have caused passenger fatalities, an anonymous whistleblower claimed on the BBC last night.

The Network Rail engineer was speaking after an independent report found that large steel girders were left dangling inches above trains going through Balcombe Tunnel in 2011.

More than 70 million passengers use the London to Brighton line each year, with 2,500 trains passing through the Victorian era-built tunnel each week.

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The whistleblower said: “You’re looking at a fatality if that steel bar would have come down. God forbid, if it had gone down between two trains then it would have gone through both, ripped the train. There would have been fatalities.

“These tunnels should be inspected more than what they’re doing now and with competent people. No disrespect, but not someone out of university or college with a degree in one hand and a torch in the other.

“They haven’t got enough men on the ground as it is to do a track examination. What used to be examined three to four times a week is now examined in some parts once a week or once a fortnight. That’s a little bit worrying.”

Mick Whelan, general secretary of the ASLEF train drivers’ union, said: “I am disgusted and dismayed to learn that the engineer responsible for 120 tunnels, including Balcombe, was not suitably qualified and was not given the correct support.

“The travelling public, and those of us who work on the railway, have been badly let down.”