Broadband, TV, mobile and landline phone companies will have to tell customers when their contract is coming to an end and show them the best deals available under new rules.
Telecoms regulator Ofcom announced the move as part of its fairness for customers programme.
The change means that from next year, providers must send tailored information to millions of individual customers, including the contract end date, the price before this date, any proposed changes and the best deals offered by their provider.
The new rules could see up to 20 million customers benefit by switching providers or agreeing a new deal with their current one.
Ofcom said people who bundle their landline and broadband services together pay on average around 20% more when they are out of contract, rising to 26% among customers who bundle in their pay-TV services.
It said that around one in seven customers did not know whether they were still tied to the original deal.
Ofcom’s consumer group director, Lindsey Fussell, said: “We’re making sure customers are treated fairly, by making companies give them the information they need, when they need it.
“This will put power in the hands of millions of people who are paying more than necessary when they’re no longer tied to a contract.”
Ofcom said the new rules would not come into force until 15 February next year, giving companies nine months to make the necessary changes to their systems and processes to make sure they “get this right”.