Ofgem has warned it will investigate and take potential legal action against energy firms forcing households on to prepayment meters, which has risen sharply.
The Citizens Advice Bureau estimates that 3.2mln people ran out of credit on their prepayment meter last year.
Energy suppliers have been under pressure to stop switching vulnerable households to this method of payment.
Business Secretary Grant Shapps warned on Sunday that he would “name and shame” suppliers who were doing “nowhere near enough” for vulnerable customers.
The government energy support scheme, which currently caps the average household bill at £2,500, is set to rise to £3,000 in April when the relief ends.
Data from energy consultancy Cornwall Insights predicts the Ofgem cap will fall to £2,200 by July, below the government relief scheme.
A survey by Which? found that Shell PLC (LSE:SHEL, NYSE:SHEL) and SSE PLC (LSE:SSE) were among the lowest-rated energy suppliers by customers, with Scottish Power ranking bottom for satisfaction.
Centrica PLC (LSE:CNA)-owned British Gas ranked ninth, with a score of 60%.
Topping the chart was Octopus Energy, with a score of 78%, with Utilita in second at 67% and Utility Warehouse third at 65%.