
The energy regulator Ofgem has told companies to stop charging customers different rates if they pay by direct debit or pre-payment meters.
It added it was unfair that 4.3 million customers with no gas supply could not get the best deals on electricity.
Ofgem said the energy market worked well for most customers, but told power companies they needed to deliver the benefits of competition to all.
It found no evidence that firms had been working together to set prices.
Ofgem started the investigation in February after all the leading suppliers imposed big price rises at the same time.
There will now be a consultation on the report, which will end on 1 December.
The regulator pointed out that competition could not stop rising prices of oil and coal from pushing up bills.
However, it said energy firms could do more to make sure that all customers were able to access the best tariffs.
"Initial findings from our energy market probe give us grounds to demand that companies end practices that hinder customers, especially the vulnerable, from getting the best deal," said Ofgem chief executive Alistair Buchanan.
"If they fail to satisfy our requirements voluntarily then we can move to a Competition Commission reference," he threatened.
There has been widespread criticism of the billing system for customers on pre-payment meters, who are charged more for their fuel than those who pay by direct debit.