Broadband operators are hammering customers who live in "broadband blackspots", charging them up to £15 a month more than other users, and an Ofcom review could increase costs even further, according to research.

Twelve million people live outside Tiscali's network coverage area and must pay an extra £8 per month for broadband, while AOL's blackspot covers nearly 10 million homes and these customers are charged an extra £10, said the study by price comparison web site Moneysupermarket.com.

Blackspots exist where an ISP does not have its own equipment installed in the local exchange and has to buy connectivity from BT on a wholesale basis, which typically costs more.

And the situation is about to get worse, as Ofcom prepares to review charges, said Rob Barnes, head of broadband and mobiles at Moneysupermarket.com.

“Millions of people are penalised for living in these broadband blackspots. Major household names such as AOL, Tiscali and Talk Talk charge extra for those outside their network and as a result pass on costs of up to £15 per month," he said.

“With the Ofcom review of BT’s Openreach charges due at the end of the year, costs are certain to rise. Ofcom should be ensuring providers accessing wholesale areas are charged a fair amount in order for the provider to then pass on the best value deal to the consumer."