Ofcom, which had first announced the ban in October, said this should make it easier for consumers to move to a different network with their existing handset.
All phone companies will have to adhere to the new rule, including O2, Three and Sky Mobile.
“Some companies have still been selling mobile phones that can’t be used on other networks, unless they are unlocked. But this can be complicated for customers and cost around £10 ($13) too,” Ofcom said.
Half of customers who try to unlock their phone have difficulties doing so, including long delays or loss of service, the regulator found.
Its research also showed that more than a third of people who decided against switching mobile networks said having to get their phone unlocked is what put them off
“This means they could be missing out on a better deal,” said the communications regulator.
Any mobile customers who want to change their provider can now use the text-to-switch process, where they get a code by sending a free text message.
Ofcom has also introduced other rules to benefit consumers.
If a customer adds a service to their package, providers will not be able to extend the contract periods of the existing services the customer already has without their consent.
This will help give customers more flexibility to switch package or provider, without being locked into long deals, Ofcom said.
It has also strengthened requirements on phone and broadband companies to provide blind or vision impaired customers with bills and contract information in accessible formats, such as braille or large print.
Disabled customers can now request that any important information about their service — excluding marketing materials — is provided in a format that meets their needs, at no extra cost.
“These rules are part of a package of changes we are introducing, to help ensure phone and broadband customers are treated fairly and they can find the best deal for them with ease,” Ofcom said.