Thousands of families were left without power for more than four hours last night after thieves made off with a vital piece of equipment from a substation.

Power was reportedly cut for safety reasons last night after a transformer was stolen.

Homes in Watford and Croxley Green endured an evening without light, businesses were forced to close, and firefighters rushed to numerous fruitless call-outs as fire alarms across the area malfunctioned.

Pam Dajda from Hertfordshire Fire and Rescue said fire crews from Watford, Garston, Rickmansworth and Hemel Hempstead were "fully stretched" during the power cut, especially between 6.30pm and 8pm.

During this time firefighters were called to 11 false alarms and had to rescue people from two lifts in Clarendon Road in eight minutes.

Drivers also faced a perilous commute home as traffic lights on many routes out of town failed.

Hertfordshire Highways representative Jo Brown said the traffic lights shut off by the power cut included those on Rickmansworth Road near Shepherds Road, Rickmansworth Road near Cassio Bridge Road, St Albans Road junction with Lowestoft Road, Beecham Grove junction with Lower High Street, Cassio Road junction with Whippendell Road and Exchange Road junction with Market Street. Street lights in these areas were unaffected as they run through a different circuit to the traffic lights.

She added that back up generators for the county's 600 pairs of traffic lights were not financially viable.

EDF representative, Tracey Sparling, said:"Unfortunately EDF Energy had to isolate supplies to several thousand customers in Watford at 6.22pm yesterday because vital earthing equipment was stolen from four 132,000 volt grid transformers.

"We do apologise to customers for the inconvenience this has caused but for safety reasons we had to immediately isolate supplies.

"Essential safety work and repairs took place, and we were able to restore most supplies by 1.40am today.

"A small number of customers remain without power and we are currently investigating the numbers involved. We believe this could be due to a secondary fault in the same area.

"Those behind these thefts risked their lives - as well as the safety of anyone who may have been close to the equipment after the theft. We will not tolerate such reckless acts and will be working closely with the police."