About 10% of gas prepayment customers are in debt

Suppliers have agreed to end the practice known as "debt-blocking" for prepayment customers who owe less than �100.

From February 2004, firms will no longer be able prevent these customers from transferring to a cheaper supplier.

But Energywatch, the consumer watchdog, said the scheme would do little to alleviate fuel poverty.

Transfers blocked

Most consumers can switch their energy supplier and save money, but many of the poorest customers are prevented by their suppliers from switching to get a better deal.

This is because suppliers block transfers when a customer is in debt, a process known as "debt-blocking".

Only a relatively small percentage of those in debt will be able to benefit Ann Robinson, Energywatch

Under the new scheme, people with less than �100 debt can switch supplier, and transfer their outstanding debt with them.

Ofgem said up to a million customers could benefit from the change in the rules.

But Energywatch said the initiative would do little to improve "the lot of Britain's most vulnerable consumers".

Chair Ann Robinson said: "The average debt among energy consumers is �150, so only a relatively small percentage of those in debt will be able to benefit from this proposal.