Some mobile networks, including BT, EE, Tesco Mobile and Vodafone, sell mobile phones that can’t be used on other networks unless they are unlocked.

But this is a potentially complicated process which normally costs about £10. Ofcom research found that more than a third of people who decided against switching said having to get their handset unlocked would put them off.

Almost half of customers who try to unlock their phone have difficulties doing so.

For example, they might experience a long delay before getting the code they need to unlock their phone; the code might not work; or they could suffer a loss of service if they did not realise their phone was locked before they tried to switch.

The rule change aims to make it easier for people to switch mobile networks as they will be able to move to a different network with their existing handset, hassle-free. The new rules will come in to effect from December 2021.

Selina Chadha, Ofcom’s connectivity director, said: “We know that lots of people can be put off from switching because their handset is locked. So we’re banning mobile companies from selling locked phones, which will save people time, money and effort – and help them unlock better deals.”

The ban on selling locked handsets is part of a package of measures Ofcom is introducing, most of which reflect new European rules.

Richard Neudegg, head of regulation at Uswitch.com, said: “Today’s announcement on mobile handset locking will finally rid the industry of this anachronistic practice.

“From December 2021, mobile providers which have continued to sell handsets locked to just one network, will no longer be able to do so, consigning the fiddly ‘unlocking’ process to the history books.

“Despite some modest improvements to the process, unlocking, when required, is often a pain – with Ofcom’s data showing that nearly half of customers who go through it experience some sort of difficulty.

“When the new rules come into force, customers will be able to buy the phone and package they want, with whatever network, safe in the knowledge that if they later choose to switch to another network, they can do so easily and base the decision purely on what’s right for them.”