Consumers can now leave their telephone and internet provider mid-contract if their bills are increased, following a clampdown by regulator Ofcom.
Some providers have been raising charges during fixed-term contracts, which typically span 18 or 24-months, with Ofcom receiving more than 1,000 complaints about the practice.
A total of 10 companies, including Vodafone, BT, Sky and Virgin Media, had put up prices having offered a "fixed-price deal" when customers took out the contract.
Ofcom has been consulting on how to protect people on contracts from unexpected price rises, with plans proposed in October to enable consumers to move supplier without penalty. These take effect from 23 January.
Providers must give 30 days' notice that they are increasing prices, and customers must be able to cancel their contracts during this period without incurring a penalty.
The rules will apply to any new landline, broadband and mobile contract, including some bundled contracts, entered into after this date, and also apply to small businesses of up to 10 employees.
The same applies if the provider reduces the number of minutes or the amount of data provided, rather than actually increasing the charges.
Pay TV will be included if it is part of a package, but not where it is sold on its own.
Claudio Pollack, consumer group director of Ofcom, said: "We have reached an important milestone in our work to ensure consumers and small businesses have better protection against unexpected price increases."
EU law meant the regulator was unable to completely ban price rises mid-contract.
Richard Lloyd, executive director of Which?, said: "This is good news for mobile phone customers and the 60,000 people who supported our campaign against unfair price rises in fixed contracts. People can now be confident that the price really will be fixed when they sign a mobile contract, or they can walk away without a steep penalty if faced with a hike.
"We will be keeping a close eye on the mobile phone industry to make sure it follows this guidance and gives customers clear information about contracts at the point of sale."
Ofcom has also produced a checklist to help guide customers on what to consider when they are taking out a new contract.
