Mobile phone companies have been accused of making £90m a year by legally using "hidden" contract clauses to raise the cost of fixed-rate tariffs during customers' contracts.
The consumer group Which? has filed an official complaint to regulator Ofcom after an investigation found that 70% of people on fixed contracts did not know that network providers could increase prices during the lifetime of their contract.
It has launched a campaign, Fixed Means Fixed, calling for an end to price increases on mobile phone contracts, arguing they should stay the same price from start to finish of the contract.
Richard Lloyd, the executive director of Which?, said: "These hidden price rises mean millions of people are forced to pay more than they expected at a time when household budgets are already squeezed.
"They are then trapped in a contract, unable to switch to a cheaper provider without paying a hefty penalty.
"Ofcom must intervene now and stamp this out. Consumers must be confident that fixed really does mean fixed."
Which? said companies including Three, Vodafone (LSE: VOD.L - news) , T-Mobile and Orange were all raising prices, saying Three had just increased its fixed-tariff prices by 3.6%, a move expected to affect more than one million customers.
The consumer group said it had been contacted by more than 1,700 people regarding price rises and called for price and all other aspects of fixed deals to remain the same for the contract period when consumers are tied-in.
If there is a chance that prices may rise, operators must be clearer about this in their advertising and allow people to switch providers without penalty, Which? said.
On its website, Three said it was increasing the cost of phone and mobile broadband contracts set up before March 8, 2012 by 3.6%, in line with the Retail Price Index (RPI) measure of inflation.