Central Networks said offenders were breaking into sub-stations and ripping out earthing cables, in response to a rise in the value of copper.

Spokesman Jamee Majid said thefts rose threefold over the past 12 months.

He urged the public to contact the authorities if they saw anyone acting suspiciously near the sites.

"It is a reckless activity as it endangers the lives of the thieves and it inconveniences customers who have their power supplies cut.

"We would encourage people who see anyone acting suspiciously, people who shouldn't be there and are not wearing E.ON UK - Central Networks branded outfits, to contact the police," said Mr Majid.

A 16-year-old boy died when he was electrocuted inside a sub-station in Gainsborough, Lincolnshire in November 2006.