An estimated 15 million households are overpaying for their broadband by around £100 a year by languishing on expired contracts, a survey suggests.
Out-of-contract prices have increased by 38% for copper and 19% for fibre broadband while in-contract prices have changed barely at all since 2011 - just £1.12 for copper broadband and 54p for fibre.
This means those who have not moved to new contracts are collectively overpaying by £1.5 billion a year, uSwitch.com said.
Consumers on expired deals are paying an average of £105 a year more than those who are on a current contract, while fibre broadband customers who are out of contract are overpaying by an average of £79, the comparison site calculated.
The comparison site said it estimated that around 15.5 million households had not switched in the last three years due to a combination of simply forgetting, underestimating out-of-contract charges, confusion or a fear of broadband downtime, and were therefore paying the unnecessarily high prices.
Meanwhile, line rental, which must now be advertised as part of the overall price of a new broadband contract, has increased by 37% in five years, meaning it is £58 a year more than in 2011.
Both Sky and BT recently announced broadband price increases.
USwitch broadband spokesman Ewan Taylor-Gibson said: "A combination of soaring line rental costs and out-of-contract pricing has really hammered broadband users over the past five years. The good news is you don't have to put up with it.
"Check your bill and make absolutely sure you're still in contract, because anyone who isn't is likely paying through the nose.
"Most broadband contracts are 12 or 18 months long, so if you've not moved in the past year, you might well find you're out of contract and paying, on average, 38% more than everyone else."