THIEVES trying to steal valuable copper cables may have been responsible for a blackout that plunged 50,000 homes and businesses in Scotland's largest city into darkness and caused massive disruption on the roads, it has emerged.

Police have launched an investigation after high- voltage electrical cables were set on fire and badly damaged on Monday night, cutting power to a large area on Glasgow’s south side. Detectives are examining the theory that someone was trying to steal the cables to sell as scrap metal, which has become highly valuable as the international price for copper has boomed during the recession.

To steal metal cables, thieves first have to burn off the plastic or polymer coating before cutting the wire inside. However, this is extremely dangerous and there have been 10 deaths related to the practice in the last year.

Similar thefts have plagued rail and telecommunications operators in recent years, and a dedicated police taskforce has been set up to crack down on the crime.

However, while the problem has so far caused around 15,500 hours of delay for trains and phone lines being cut, there has been nothing on the scale of Monday night’s disruption.

As well as the blackout, smoke from the blaze in an underpass on the M8 near Kingsinch Drive, Renfrew, caused the motorway to shut for four hours, bringing long delays for drivers.

Last night, Detective Sergeant Gordon McCrae, of Paisley CID, said: “From our inquiries so far, it would appear that the electrical cables in the disused underpass were set alight, which resulted in damage and disruption.

“Why they have been set on fire has still to be established, but it was an incredibly dangerous and very selfish thing to do.”

ScottishPower said Monday’s fire had caused “major damage” to the 132,000-volt cables, a key part of the local network.