An energy watchdog has launched a "super-complaint" into the way in which energy suppliers charge their customers.

Last year 40,000 people called the gas and electricity watchdog energywatch to complain about their energy bills.

The numbers of complaints and inquiries about bills soared by 26 per cent between 2002 and 2004.

Over the next 90 days, regulator Ofgem will look into the energy suppliers and the way they handle customers' bills.

The "super-complaint" - which arises when organisations file complaints about whole markets - is thought to be a first for the energy sector.

Allan Asher, chief executive of energywatch, said: "Billing is the single biggest problem consumers have with energy suppliers.

"The industry has consistently resisted any reform, which is why we hope Ofgem will force through a radical overhaul of the billing process.

"Figures given by suppliers to energywatch confirm the shocking number of problems caused by billing." Pressure to reform the billing process has also come from MPs.

Environment Minister Michael Meacher said: " Without accurate energy bills, consumers have no idea if they are being energy efficient.

"For this and many other reasons, I welcome the supercomplaint by energywatch and trust Ofgem will take their obligation to protect consumer interests seriously."

Problems that have arisen include failure to deliver bills for months and sometimes years, bills being wrongly calculated and companies relying on estimates which are often incorrect.

Mr Asher launched the super-complaint in London yesterday flanked by two women pensioners who have been wrongly charged to the tune of several thousand pounds by their energy suppliers.