A broadband comparison site has warned of the sometimes "expensive consequences" for customers who sign up to unbundled providers. Local Loop Unbundling (LLU) has been hailed as an evolution in cheaper broadband.

Unbundled providers are companies that control your phone line as well as your broadband. This means you pay your line rental to one company, cutting out BT .

"It costs somewhere in the region of £35,000 to unbundle a single exchange," says Michael Phillips, product director at BroadbandChoices.co.uk . "So providers are keen to add as many customers as possible to their unbundled networks - sometimes moving them over without the customer even realising."

"But because LLU uses a different type of technology to traditional ADSL connections, customers could face a break in their broadband connection and have difficulty in returning to the BT network if they're not happy with the service," Phillips explains.

Users who sign up to unbundled services could face a break in their broadband service. "They may even have to pay BT £124.99 to have their phone line reconnected if both their broadband and home phone had been moved to an unbundled network," added Phillips.

LLU services

BT says around 1,000 exchanges have now been unbundled by providers, while there are around 2.5 million customers using LLU services. BroadbandChoices.co.uk says a service is being trialled to enable LLU customers to switch providers much more easily. "We need increased clarity from ISP's and definitive regulation from Ofcom so that these problems do not persist," says Phillips.

Ofcom introduced new legislation in February to prevent broadband providers from stalling when users wanted to leave their service. ISPs now have five days to surrender the MAC (Migration Authorisation Code) necessary to switch service.