Vodafone, EE, Three and O2 have been named in a writ claiming they made billions by overcharging overcharged customers for phone handsets beyond the end of their contracts.
Justin Gutmann, a former executive at Citizens Advice, has brought the £3 billion legal action on behalf of 4.8 million mobile phone users.
Gutman calculates that 28.2 million UK mobile phone contracts could be affected over a period dating back to 2007.
If he wins, customers stand to receive up to £1,800 per operator with which they had a contract.
The claim alleges that contracts for mobile phones usually include the cost of the handset as well as services used, but once the minimum payment term ended the amount charged didn’t change even if the handset had been paid for.
That meant users being charged more than a new customer on a sim-only deal, Gutmann said.
"If our claim is successful, it will finally stop these firms from taking advantage of their loyal customers and stop the immoral practice of loyalty penalties,"
All mobile consumers are automatically included in the claim for free unless they take steps to opt out.
EE, owned by BT, told the BBC the claim was "speculative", adding it "strongly disagreed” with the accusations.
"EE offers a range of tariffs and a robust process for dealing with end-of-contract notifications."
Other operators said it was too early to comment or had yet to be contacted.