Ofcom, which first pitched the idea in December last year, said the move will be implemented from December 2021 to make it easier for consumers to switch providers.
He said the rule targets companies still selling devices that cannot be used on other networks unless they are unlocked for a fee – usually around £ 10.
They included, said Ofcom, BT / EE, Tesco Mobile and Vodafone, while O2, Sky – the owner of Sky News – Three and Virgin all chose to sell unlocked handsets.
The watchdog said its research found the issue prevented more than a third of people from switching phones with their existing phones and potentially getting a better deal.
He also cited time-consuming challenges faced by many consumers who currently need to be given a code to unlock their device.
Ofcom said the frustrations included failed codes and loss of service during the unlock process.
This decision builds on the regulator’s previous work to facilitate the switch of supplier, which included the ” text-to-switch « a service.
Its Connectivity Director, Selina Chadha, said: “We know that a lot of people can be deterred from switching because their handset is locked.
“So we’re banning cell phone companies from selling locked phones, which will save people time, money and effort – and help them get better deals.
Ofcom said a broader set of measures would support the process of change.
This meant that mobile and broadband operators were forced to send potential customers a summary of their contract terms before a sale.
The regulator said it would also organize a consultation on a simpler process for switching broadband providers.