The communications regulator said it was likely to regard any increase to a recurring monthly charge in a fixed-term contract as "materially detrimental" to consumers, and said providers should give at least 30 days' notice of any price rise and allow customers to leave without penalty.
Any changes to the contract, pricing or otherwise, "must be communicated clearly and transparently" to consumers.
The announcement follows a consultation on how to give consumers, including small businesses, a fairer deal in relation to price increases.
The new guidance comes into effect in three months and will apply to any new landline, broadband and mobile contracts that start after this time.
Which? welcomed the announcement as an "overwhelming victory" for consumers.