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Cannabis farms are sucking as much as £70 million worth of electricity from the national grid each year, a report warned today.

The power needed to illuminate the plants as they are grown indoors accounts for about a third of all the electricity stolen each year, according to watchdog Ofgem.

It means the typical householder has to pay an annual £2 “weed levy” on their electricity bills to cover the cost of the theft, it said.

Ofgem, which estimates there are around 25,000 cases of electricity theft every year, today put forward tough new rules to encourage suppliers to take action to prevent cannabis growers diverting electricity. This could include fines if suppliers do not investigate swiftly.

Chief executive Andrew Wright said: ”Ofgem wants to make sure that consumers are paying no more than they need for their electricity, and lives are not put at risk.

“It’s critical that suppliers do all they can to clamp down on electricity theft. That is why Ofgem is introducing new rules to encourage better trheft detection.”

But Angela Knight, head of Energy UK, the trade association for the electricity and gas companies, told BBC Radio 4’s Today programme that while the industry was keen to “step up to the plate” she was unhappy about the threatened fines.

She said: “It’s a bit of a pity they’ve done it that way, but working with Ofgem to get this right, we’re absolutely up for that.

“This is complicated, (the cannabis growers) are bypassing the system, so the normal meter reading, you don’t pick it up.”

The number of suburban cannabis farms in otherwise ordinary looking residential premises has soared in recent years with about 20 being discovered by police everyday.

Operators, usually Vietnamese or Chinese gangs, are also often linked with the human trafficking trade with young Asian boys forced to work as virtual slaves in the farms, which use around 40 times more electricity than normal households.

Tell-tale signs include permanenty shuttered or covered windows and large numbers of empty compost bags dumped by bins. A typical farm of 500 plants will need around 80 lights switched on for at least 12 hours out of every 24.

Cannabis is a ‘class B’ drug in Britain, which means those who supply and produce it can be imprisoned for up to 14 years and/or face an unlimited fine.

Under the new proposals, suppliers will be asked to set up a national risk assessment service to help them target premises where there are strong suspicions that electricity is being stolen, most commonly through tampering with meters.

There will an industry code of practice governing how theft investigations should be carried out and a 24-hour hotline to report suspected theft.

The deadline for responses to Ofgem’s proposals is 28 August.