Criminal gangs are helping homeowners and landlords avoid paying for power by "hotwiring" supplies for as little as £10, BBC Inside Out has learned.
The energy thieves operating in London can tamper with 15 meters in a day, an investigation found.
Fraud investigators from British Gas said the gangs were risking causing gas explosions and endangering lives.
Regulator Ofgem believes theft costs the industry £400m per year. British Gas says it adds £30 to annual bills.
Mark Andrews, the firm's head of revenue protection, has warned people die "every year" because of meter tampering.
Power theft investigator Piers Merritt said: "We find trends where people go around estates and tamper with meters.
"These people will charge between £10 and £500 a time.
"They're not putting themselves at risk, if something happened there it's all the adjoining properties and everyone in there that's going to be affected by a potential explosion."
Investigators found five instances of gas theft and two of electricity theft in a Sheffield street where a house was destroyed in a suspected gas explosion last year.
The scale of the damage meant the cause of the explosion could not be identified.
Mr Andrews said: "The police were on site and they asked us whether or not we felt it was right to look at some of the other supplies on the street and yes, from 20 houses in a single terraced street, sure enough we found five gas thefts and two electricity thefts.
"It's a great example what the scale of the issue is."
In London, a man who received a suspended prison sentence for digging up a street in the capital, allowing residents to steal electricity, told Inside Out "thousands" of homes in the capital were avoiding paying for usage by "hotwiring" their supply.
Ofgem told the BBC it would be publishing its decision on the new obligations for energy suppliers to be more proactive in tackling electricity theft in February. It introduced similar obligations for gas suppliers during January 2013.
Mr Andrews said: "The difficult economic climate equals more people feeling this is the only way they can get their energy.
"This is a huge problem, we believe that perhaps £500m worth of gas and electricity is stolen across the industry each year.
"In terms of what that means for the customers, it's potentially £30 a year on their bills and it's money they shouldn't have to pay."
Inside Out was shown examples of bicycle inner tubes being used to divert supply.
Lewisham Council has uncovered evidence that landlords are stealing the supply for multiple properties on the same street.
Ben Reeve-Lewis, from the council, said he regularly visited homes where meters suggested no electricity had been used for three months.
"Landlords of that kind will fill up their properties with people who can't complain," he said.
Ofgem believe a third of energy theft is linked to the illegal growing of cannabis and police and energy firm investigators have found booby traps close to illegitimate equipment.
Insp Lee Devall, from Essex Police, said: "Organised crime gangs don't want you to find this and are not happy when thousands of pounds worth of equipment is ruined."
Inside Out is broadcast on Monday, 20 January at 19:30 GMT on BBC One and is available for seven days thereafter on the iPlayer.