Last night’s wide-ranging power cut, which left thousands of homes without power, was caused by scrap-metal thieves, it has been confirmed.

EDF Energy says it was forced to cut power for safety reasons when high voltage cables were stolen from a substation in Vicarage Road.

Customers in the West Watford, South Oxhey, and Northwood areas were all affected by the outage, which occurred when the thieves struck at about 9pm.

Firefighters were also forced to make many needless trips when fire alarms sounded because of the black-out.

Last night’s theft was the second in the local area in as many years. Thousands of households in similar areas were affected In June 2008 after a similar attack.

An EDF spokesman said: “Anyone entering a substation, other than our own trained engineers, is risking their lives and the lives of others.

“Our biggest concern is that thieves risk being electrocuted if they come into contact with high voltage electrical equipment. We will press for the prosecution of anyone caught tampering with our networks and will work with the police to help their investigation of this crime.”

“We would ask customers to report any suspicious activity to police or our Substation Watch hotline, 0800 587 3243.

“Metal theft is currently a global problem, driven by the rising price and high demand for metal across the world.”

Hertfordshire Constabulary is investigating the crime.