Gas and electricity companies often treat customers who question their bills "like criminals" or simply ignore them.

Energywatch said it has examples of customers being referred to debt collection agencies or disconnected when they were wrongly accused of owing money.

According to the watchdog, almost half the 86,000 complaints it received last year related to billing and accounts.

It had heard of energy suppliers chasing the wrong customer for payment, of people being disconnected over "grossly over-estimated" bills and of customers being hassled by debt collection firms.

Energywatch warned suppliers were breaking their statutory obligations and said it would consider asking the energy regulator to take action.

Chairwoman Ann Robinson said: "Companies say they care about their customers so why aren't they listening to them?

"If a customer is questioning a bill, the company must accept those concerns as legitimate and investigate thoroughly.

"What we don't expect are companies employing bully boy tactics to force people to pay up even when a bill is is question."

Energywatch said 1,000 electricity customers were disconnected last year, up from 375 in 2001.

Gas disconnections fell from 26,000 to 21,000 during the same period.

British Gas said: "It is quite difficult to get yourself into a position of being disconnected. We work hard to try to prevent customers getting into debt.

"We only resort to disconnection where we cannot agree a repayment arrangement with the customer - usually because they won't respond to our messages."

British Gas came under fire a second time yesterday after it was fined £200,000 for stopping customers switching to rival suppliers.

An investigation by energy regulator Ofgem found the company raised objections to almost 5,000 customers who paid by direct debit changing to another firm because it claimed they were in debt.

Under their operating licences, suppliers can only prevent customers who owe money from switching under certain circumstances, which excludes direct debit.

British Gas has accepted Ofgem's decision, claiming a small number of staff had "incorrectly applied" written procedures on the customer transfer process.