Thieves steal cables for the copper inside them, and then sell the metal on as scrap.

Supt Mark Cleland, from British Transport Police, said: "All metal theft is primarily driven by the price of metal so, as metal rises in value, we see a trend that crime rises with it. At the moment we're in this upward trend of the price of metal rising."

Experts say that even if the cable is security-marked, it can be made untraceable by stripping the rubber and granulating the metal at scrapyards.

'Extremely dangerous'

James Nattrass, director of incident management and operational security at Network Rail, said cable theft was "not a victimless crime".

"It costs the taxpayer millions of pounds a year, and the total cost to the economy is even higher when you consider the impact of delays to freight, and to passengers who want to get work.

"Not only is it disruptive for our passengers, it is also extremely dangerous for the perpetrators. Thousands of volts of electricity run through cables and interfering with them can be fatal."

The Scrap Metal Dealers Act was introduced in 2013 to try to clamp down on metal thefts - with cash sales banned and all dealers needing a licence.

But a Freedom of Information request showed that last year in England, a third of mobile scrap collectors had not renewed their licences.

The Local Government Association defended the act, saying: "A drop in the current levels of renewals could be for a number of reasons, not least one being that the act has subsequently discouraged those businesses who were not operating within the law."

A Home Office spokesperson said the act "continues to play a fundamental part of our efforts to tackle metal theft by removing the opportunities for criminals to dispose of stolen metal".

Source: BBC