Energy companies have been accused of using customers' money for interest-free loans by setting monthly direct debit payments too high.

Four out of five people pay for their gas and electricity by monthly direct debit, but 70% of these people are in credit to their power company, according to Which? Money.

It found that a quarter of people in credit were owed more than £100 by their energy supplier, with 8% owed more than £200.

The group said the high levels of credit some people had built up were particularly surprising, as the research was carried in winter, when energy use is typically higher.

It warned that customers were losing out on interest they could earn on the money they had overpaid to power companies.

Martyn Hocking, editor of Which? Money, said: "It seems incredible that energy companies can take hundreds of pounds more than they need to from their customers, and profit from the interest that this money will earn at our expense.

"While a small amount of credit built up over the summer months can be used up during the winter, it's difficult to see how a £200 credit will be used up - particularly as the customer makes the same payment each month."

Which? Money urged people to regularly check their meter reading and the level of direct debit they were paying, and contact their energy supplier if they thought their monthly payments did not reflect their usage.

The group also found that having their direct debits set too high was the most common complaint people made about energy providers.

Its latest customer satisfaction survey found that energy suppliers were one of the lowest performing sectors when it came to keeping customers happy, with an average consumer satisfaction score of just 43%, down from 47% when the same research was carried out last year.

Npower received the lowest score for an energy company in terms of customer satisfaction at just 30%, closely followed by British Gas at 37% and E.ON at 39%, while Utility Warehouse got the highest score at 83%.

An npower spokeswoman said: "We're disappointed with these findings as we believe paying by direct debit allows customers to manage their budget.

"It is also common for customers to owe money to their energy company in the winter and to be in credit at the end of the summer period.

"We assess customer accounts twice a year - if a customer's credit balance is in excess of £60 at the time of their assessment, this will be automatically refunded."

An E.ON spokeswoman said: "We are working hard to improve all elements of our customer service and are disappointed to have slipped down the Which? rankings. We have already reduced electricity prices this year and are working to make our bills easier to understand.

"For customers who pay by direct debit, we review their payments annually to check whether there is a credit or debit on the account, as well as aligning their payments quarterly, with the aim of ensuring a customer's balance is zero in the spring.

"We automatically provide a refund to customers who are in credit in the spring if we have actual meter readings."

Which? questioned 6,088 of its panel members during November and December.