The recent cuts in energy bills could leave utility companies with millions of pounds of their customers’ money in their bank accounts because they have not updated their direct debits.

Consumer groups have also pointed out that the foreign owners of many UK utilities companies automatically rebate overpayments to customers in their own countries while failing to do so here.

Critics have pointed out that while most firms conducting business send a bill and then collect the money they are actually owed, energy companies take the same amount every month regardless of how much energy a customer has actually used.

A warm winter and an exceptionally hot April have meant that many people have used a lot less energy this winter, but most energy companies only review payment levels once every six months so they often have a lot of their customers’ money in their bank accounts.

A spokesperson for the Government watchdog Energywatch said: “It is impossible to get accurate figures on just how much cash these companies are sitting on, which is a contentious issue in itself.

“Never take a bill at face value as companies cannot be trusted to give you an accurate bill unless you take a meter reading.”