Loyal home insurance customers can find themselves paying a £75-per-year loyalty penalty for sticking with the same provider, Which? has found.
The consumer group found customers with combined buildings and contents policies owned for longer than a year were paying, on average, £75 more annually than new customers.
On average, existing customers were paying £270 per year for a combined policy, while new customers were paying £195, it found.
The loyalty penalty was found to balloon over time.
Customers who had been with the same insurer for over 20 years paid around double the amount of what new customers paid.
For combined insurance, the average premium paid for a policy 20 or more years old was £396 per year, compared with the £195-per-year average for new customers.
Seven in 10 (69%) of those surveyed had been with their insurer for longer than a year.
Which? heard from one customer who was paying £554 a year for her home insurance after six years with her provider.
She spotted the same policy was available to new customers on a comparison website for £335, prompting her to cancel her policy and reapply to access the new customer discount - saving £219.
Which? said that, surprisingly, many insurers do allow customers to cancel their existing policy to take out a new one at a cheaper price.
Harry Rose, Which? money editor, said: "It is unacceptable that longstanding policyholders are taken for granted by insurance providers and hit by these excessive premiums.
"Customers who prefer to stay with one provider are at risk of being exploited by these vastly overpriced premiums when little has changed in the service they receive.
"Insurers must make sure existing customers remain a priority, and are not sidelined by the push to attract new business."
Over 7,000 people with a home insurance policy were surveyed.
A spokesman for the Association of British Insurers (ABI) said: "We recognise that the insurance market is not working as well as it should for many longstanding customers.
"We want everyone to get the best deal in a competitive market, which is why we have launched an industry initiative to help those customers who do not shop around.
"This will include insurers reviewing the policies of customers who have been on their books for five or more years to ensure they are getting the best deal for their needs."