The energy regulator Ofgem has urged suppliers to help customers before they get themselves into unmanageable levels of debt.
New figures show customers struggling to pay their gas and electricity bills build up an average of £600 in debt before providers intervene.
Although the overall number of customers in debt to their suppliers has fallen, some suppliers are letting vulnerable users build up higher arrears before stepping in.
The number of customers in debt to energy firms fell by 9% to 971,362 for gas customers and by 3% to 1,195,635 for electricity customers over the past year.
But supplies let average electricity debts increase by 7% to £628 per household before taking steps to put customers on a debt repayment progamme.
Average gas debt increase by 5% to £622, Ofgem said.
Npower, Utility Warehouse, Ecotricity, iSupplyEnergy and First Utility were criticised for letting customers build up £800 on average in electricity debt before they start paying it off.
NPower, Utility Warehouse and Spark Energy also let customers build up an average of £800 in unpaid gas bills before they intervened.
Rachel Fletcher, Ofgem’s senior partner for consumers and competition, said in a statement: “Paying off energy bills is a major concern for many customers in vulnerable situations.
“When suppliers let big debts accrue, it’s a sign that they’re not spotting debt or stepping in early enough to help customers who are struggling to pay bills.
“We want industry to demonstrate that it is identifying and supporting these customers in a timely way.
“We will be monitoring suppliers to make sure they make long-term improvements on bringing down debt.”
Ofgem recently announced plans to extend its safeguard tariff for prepayment customers to around one million vulnerable households this winter to help them save an average of £120 a year.
It has also introduced a new rule requiring energy suppliers to identify vulnerable customers and take extra action to support them.
According to Ofgem, low-income households are most likely to be paying more than necessary for energy, with 40% of households on less than £16,000 say they’ve never switched providers.
Only 30% of consumers on incomes of more than £16,000 say they have never switched energy company.
The Office for National Statistics found that in 2016, low-income households spent 9.7% of their budget on energy bills while those on the highest incomes only spent 2.9%.