Citizens Advice has found customers who remain loyal to their utility providers are being penalised by £4 billion a year.
The charity has lodged a super complaint with the competition watchdog.
It said the practice of overcharging loyal customers was ongoing and widespread and called on the Competition and Markets Authority (CMA) to “act now to stop people being exploited”.
Its research found 80 percent of British consumers pay a significantly higher price to at least one of their providers for remaining with them.
They are losing £4.1 billion a year across the five essential consumer markets of mobile, broadband, home insurance, mortgages and savings.
Although the Government’s price cap in the energy market would bring down loyal customers’ bills by £75 per year on average, Citizens Advice said “excessive prices” for those who remain with the same provider in other markets could be just as high, if not more so.
The charity also found the loyalty penalty is disproportionately paid by vulnerable consumers such as older people and those with mental health issues – groups that are particularly likely to struggle with switching.
It gave an example of helping an elderly couple whose daughter approached the charity after finding her parents were paying nearly £1,000 a year too much on their home insurance.
The couple, who are both in their 90s, had been with the same company for six years and over that time their premium had continually risen.
A super complaint can be made by a Government-designated consumer group to ask a regulator to investigate an issue or a market that it believes is working against the public interest.
The regulator must publicly respond to the complaint within 90 days saying if it believes it is an issue and if so how it intends to deal with it.
The CMA confirmed it would investigate the concerns with regulators such as the Financial Conduct Authority and Ofcom and publish a response within 90 days.