Millions of energy customers could be stung for one-off payments of £200 or more before they can switch supplier because of utility firms’ shambolic billing systems.

The payments are being demanded to clear debts that consumers often have no idea they have built up.

Energy giants say debts are built up in the winter months, when direct-debit customers pay less than they should because their usage goes up.

However, consumer groups say debts are being built up purely because firms’ billing systems are in such disarray. Some consumers complain they have been lured in by cheap initial rates which do not cover their usage, so they are being forced into debt.

Research by the comparison firm Uswitch (www.uswitch.com) found that 6.8m households are in debt to their energy supplier with the average debt at around £114, although in some cases this can be as high as £800.

Tim Wolfenden, energy expert at Uswitch, said: “Though most people will accumulate debts of between £100 and £200, in some extreme cases you will find people with much higher debts, leaving them unable to switch easily.

“At a time when energy tariffs are rapidly changing, it means that they could miss out on the top deals.”

The cheapest deal for dual fuel cost around £742 in January, but is now more like £866. The rapid rate of tariff changes mean that customers can no longer rest on their laurels after switching to a top deal, as it may be surpassed very quickly. In January, British Gas offered the cheapest rate, followed by Eon in February, Npower in April and British Gas again this month, with Click Energy 5.

The watchdog Energywatch said consumers should be able to switch even if they have a debit on their account, especially if this was less than £100. It said: “The supplier can demand payments upfront but if the customer can’t afford this, he should be able to request the facility of a payment method.”

Here we look at some of the most common problems consumers encounter at switching.

Direct debit debts

Direct-debit customers pay a fixed rate over the course of the year despite changes to usage. The idea is that you overpay during the summer months, when you generally use less energy, and underpay during the winter months. Over a 12-month cycle, you should end up with no debt or credit.