Electricity and gas suppliers have been warned they will be fined if they continue to send out bills to customers for money they have no right to collect.

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.

A senior boss at Ofgem, Tim Jarvis, told the BBC he was shocked and disappointed after hearing some of the cases the BBC has investigated.

Despite being banned in 2018, back billing is still happening to thousands, if not tens of thousands, of people.

Suppliers have apologised but say tens of millions of bills are sent out each month. 

Earlier this week Energy Secretary Ed Miliband wrote to the regulator urging it to “challenge unlawful back billing”.

Mr Jarvis said he has now written to the chief executives of the big energy suppliers demanding they find out the true scale of the problem, adding that the regulator will not hesitate to fine any company found to be systematically breaking the rules.

Citizens Advice received 47,000 complaints about billing in general in 2023, rising to nearly 60,000 in 2024.