Engineers said that as a result of the thefts, thousands of homes have been left without power in recent months.

The firm and Crimestoppers have offered rewards of up to �1,000 for information leading to the conviction of thieves.

But spokesman Alan Raymant said despite continued warnings, the number of incidents was still rising.

"These criminals are smashing their way into our substations and breaking into our vital equipment with little regard for their own lives or for the huge inconvenience they're causing our customers," said Mr Raymant.

"We're working closely with the police and we've put a number of security measures in place, including CCTV, to make life as difficult as possible for the thieves."

Recent incidents have involved thieves seriously damaging equipment in a Mansfield substation and intruders breaking into a primary substation in Nottingham, stealing copper earthing tape and disrupting the supply for customers.