O2, Sky, Three and Virgin already sell unlocked devices but BT/EE, Tesco Mobile and Vodafone sell handsets that can only be used on their network unless they are unlocked.

EE allows customers to unlock their device for free once their contract has ended, but others will charge as much as £10 for the privilege.

Ofcom notes that nearly half of customers who try to unlock their smartphone find it too difficult.

The watchdog growled that users might face a long delay before getting the code they need to unlock it, they get a code that does not work; or they could suffer a loss of service if they did not realise their device was locked before they tried to switch.

Lindsey Fussell, Ofcom's Consumer Group Director, said: "Switching mobile provider can be really frustrating. By freeing mobile users from locked handsets, our plans would save people time, effort and money - and help them unlock a better deal."