According to a watchdog report, loyalty to broadband households costs several hundred pounds a year.
Research from Consumer Champion Which? Most people have been with their broadband provider for at least three years and can pay almost 90% more than a new customer.
On average, those who remain loyal pay 15% more than people who attract low-cost introductory offers.
TalkTalk has the most loyal customers of the 12 largest surveyed providers, with nearly nine out of ten remaining with the supplier for more than three years.
The average increase between the price of a TalkTalk introductory offer and the standard cost of broadband after signing a contract is 15%.
However, the biggest price increase is for the company's Faster Fiber Speed Boost deal, which will rise 54% from £ 25 a month to £ 38.50 at the end of the 18-month contract period.
Alex Pridding from the broadbandchoices.co.uk website said: "This reinforces what we see over and over again – loyal customers pay a high price for trusting their supplier beyond the terms of the contract.
"Vendors will only clean up their files when their customers start shuffling their feet and compromising their bottom line, as there is currently a lot of money to be made by passive consumers."
How much do people pay more?
Eight out of ten BT customers had stayed with the provider for at least three years, although the average tariff was 23% above the average launch contract.
BT's standard broadband package will increase from EUR 24.99 to GBP 32.99 after 18 months – an increase of 32%, which costs GBP 96 a year more.
The biggest single price increase came from the post office.
The Unlimted Broadband package increased from £ 15.90, the cheapest introductory offer on the market, to £ 30 per month – an increase of 89% from £ 169 per year.
Alex Neill, which one? Home Services CEO: "Broadband customers will be outraged if they find that their loyalty reward is often a significant price increase without improving the quality of services offered.
"Suppliers should fight hard for your loyalty, do not take it for granted."
What the providers have to say
TalkTalk spokesman said: "We are committed to treating all customers fairly. We already terminate contracts before and after the end of the contract and urge customers to enter into a contract that guarantees that the broadband price for mid-contract contracts will not increase.
"Although we recognize that some customers have the flexibility to leave at any time, we reward loyalty by making great offers to those who commit to staying with us."
A BT spokesperson said: "We absolutely agree that customers should not overpay for the service they receive, and we believe that this is best achieved through clear communication with consumers about their options , We work harder than ever to be even clearer with our customers about their options, for example, when new customers can benefit from special offers, and it's very clear what the price will look like at the end of that period. "
A spokesperson for the post office said: "We always examine how we can best serve our customers. That's why we're working with Ofcom to use an industry-wide end-of-contract approach to make sure these are useful to customers.
"We want to offer a clear alternative from the big providers by offering the cheapest unbounded broadband in the UK. At the point of sale, we clearly disclose our standard prices (out-of-contract prices) to ensure that our customers can clearly see what they are paying for during and after the contract before making the change. "