Telecoms regulator Ofcom has launched a review of broadband pricing for loyal customers amid concerns they are paying more than they need to.
Ofcom figures show that 94% of UK homes and offices can now get superfast broadband, but less than half have taken it up.
Around four million households with old-style, basic broadband have passed their initial contract period and could switch to superfast for the same – or less – money than they currently pay.
Customers who take a landline and broadband service together are paying an average of 19% more once their discounted deal has expired, Ofcom found.
Sharon White, Ofcom chief executive, said: "We're concerned that many loyal broadband customers aren't getting the best deal they could.
"So we're reviewing broadband pricing practices and ensuring customers get clear, accurate information from their provider about the best deals they offer."
Ofcom announced a range of measures to help people get the best deal.
It called on broadband providers to tell customers about their best available deal, both when their deals are coming to an end and every year after that if they don't change their deal.
The watchdog is also launching a review of broadband companies' pricing practices – examining why some customers pay more than others, and whether vulnerable customers need extra protections to ensure they get a good deal.
Ofcom has already launched a website, Boost Your Broadband, that tells people what broadband they can get in their area, and offers advice on how to find the best deal.
The broadband market is highly competitive, with superfast broadband packages now available from around £20 a month, Ofcom said.
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Alex Neill, managing director of home products and services at Which?, said: "Our research has shown that many UK households are paying huge loyalty premiums for staying on the same tariff when they could be paying less for a faster internet service."
He added: "If you are unhappy with your internet service, or you think you could be paying too much, you should look to switch provider or try haggling for a better deal. A few minutes of your time could potentially save you hundreds of pounds a year."