Research by consumer group Which? found that most customers have been with their broadband provider at least three years which may mean their introductory deal has ended.

Those who are loyal pay 15% more on average than new customers, with some paying as much as 89% more, depending on the provider.

TalkTalk has the most loyal customers of the 12 major providers surveyed, with nearly nine in 10 (85%) staying with the supplier for more than three years.

But the average increase between the price of a TalkTalk introductory offer and the standard cost of broadband after the offer is over is 15%.

The biggest price increase is for the firm’s Faster Fibre Speed Boost deal, which rises by 54% (from £25 a month to £38.50) after the 18-month contract period ends – an increase of £162 a year.

Alex Neill, Which? managing director of home services, said: “Broadband customers will be outraged to discover that their reward for loyalty is often a substantial price hike, with no improvement to the quality of service they are getting.

“Suppliers should fight hard for your loyalty, not take it for granted. No one should be putting up with an internet service they are not happy with.

“Anyone who thinks they might be out of contract should look to switch to a better deal – a few minutes of your time could potentially save you hundreds of pounds a year.”

The Which? study suggests eight in 10 BT customers have been with their provider for at least three years.

The average standard BT tariff is 23% higher than the price of the provider’s average introductory deal. In the worst example, its standard broadband package rises from £24.99 to £32.99 after 18 months – a 32% hike and £96 more a year.

Sky customers face an average increase of 42% when moving onto a standard tariff once their introductory offer has ended. The biggest increase is a 50% rise from £20 to £30 a month on its Unlimited Broadband Package – £120 more a year.

When Which? looked at the possible financial implication for customers who stay with their provider beyond the end of their initial contract, it found Virgin Media customers faced the biggest price rises of the 12 broadband providers.

When they move onto a standard tariff, Virgin customers pay 52% more on average than they would as a new customer.

The supplier’s biggest increase comes after the first 12 months on its VIVID 100 fibre deal, with the price rising 74% – from £27 to £47 a month – totalling an extra £240 a year.

The biggest one-off price increase was from the Post Office. Its Unlimited Broadband package increased from £15.90 to £30 a month – an increase of £169 more a year or 89% after the 12-month contract period. The introductory offer is also currently the cheapest deal on the market.