The accusation that loyal customers are being "ripped off" for mobile, broadband, savings, home insurance and mortgage deals is to be probed by the competition body.
It follows a super-complaint from Citizens Advice, which says customers who stick with their supplier are losing a total of £4.1bn a year.
That amounts to an average of £877 per person, the charity said.
It added that elderly and vulnerable people are disproportionately affected.
The Competition and Markets Authority (CMA) said it would now "carefully consider the concerns" that Citizens Advice had raised.
Gillian Guy, chief executive of Citizens Advice, said: "It's completely unacceptable that consumers are still being ripped off for being loyal to companies they rely on every single day.
"Regulators and government have recognised the loyalty penalty as a problem for a long time - yet the lack of any meaningful progress makes this super-complaint inevitable.
"As a result of this super-complaint, the CMA should come up with concrete measures to end this systematic scam."
The charity said that while the new price cap in the energy market would cut bills for loyal customers by £75 on average, its analysis found "excessive prices for loyal customers can be just as high - if not more so - in other markets".
Citizens Advice said problems had been detected in five particular areas:
- savings accounts
- household insurance
- mobile phone contracts
- broadband contracts
It said it knew of a couple in their nineties who were paying £1,000 a year too much for their home insurance.
They had been with the same company for six years.
Banking industry group UK Finance said it would "carefully consider the issues raised by Citizens Advice".
Eric Leenders, managing director of personal finance for the body said: "The industry has already implemented a number of measures to further improve competition in the mortgages and savings market, including communicating more clearly with savers about the rates they receive and helping longstanding mortgage borrowers switch to a better deal.
"We would always encourage customers to shop around and find a deal that best suits their needs and will continue working with the regulators to make this as easy as possible, including through standard terms and price comparison tools."
Earlier this week, the telecoms regulator Ofcom announced action to stop loyal customers paying too much for their mobile phone contracts.