The accusation that loyal customers are being "ripped off" for mobile, broadband, savings, home insurance and mortgage deals is to be investigated 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.
At the same time the Financial Conduct Authority (FCA) has announced that it will investigate whether loyal customers are paying too much for home and car insurance.
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.
"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: "We would always encourage customers to shop around and find a deal that best suits their needs and we 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.
The insurance industry has also promised to make changes, after admitting that long-standing customers sometimes pay more than new ones.
It wants all insurance firms to look at customers who have been with them for more than five years.
"In a competitive free market, where three out of four people shop around, there is no easy fix available, and these measures will take time to bed in," said Huw Evans, director general of the Association of British Insurers.
Announcing its own study into the insurance market, the FCA said it had already been concerned about the issue for some time.
"We expect firms to look after the interests of all customers and treat them fairly, whether they are new or long-standing," said Andrew Bailiey, the FCA's chief executive.
"It is important to get the balance right, so that existing customers do not miss out on the benefits of competition and innovation, including when they purchase or renew their general insurance products."
This is only the fourth super-complaint that Citizens Advice has made since 2002, when the legislation came in.
The CMA now has 90 days to come up with ideas for dealing with the issue.
Possible outcomes could involve new laws, enforcement by regulators, or a full market investigation.
"Our response will set out the CMA's views on this important issue and any next steps we think are needed to make sure businesses don't take unfair advantage of their long-standing customers," said Daniel Gordon, senior director at the CMA.