The energy regulator Ofgem told BBC's Money Box that it is writing to chief executives demanding to know how many people have been sent "back bills". Back billing is when customers receive new bills for energy used longer than 12 months before - a practice that was banned seven years ago.

Standing charges are applied daily, regardless of how much energy the customer uses, and many campaigners say they affect vulnerable customers more.

A report by consumer watchdog Citizens Advice claimed that network operators made use of a regulatory loophole set by Ofgem

The energy regulator said it is consulting on bringing in the option under the price cap that would cover so-called zero standing charge tariffs

Ofgem has announced plans to help people cut their energy bills. Energy suppliers will be made to offer tariffs with low or no standing charges, meaning customers will be able to pay the costs of their unit rate instead.

Energy regulator Ofgem has issued a major update on plans to introduce new energy tariffs with no standing charges. Standing charges are a daily charge paid by all households, which cover the fixed costs of delivering energy to homes and businesses. Currently, the energy price cap dictates the maximum amount energy suppliers can charge.

Standing charges are set by energy suppliers and rack up daily, regardless of how much energy you use that day.