Regulator Ofcom said unlocking handsets could often be a complicated process, and this was discouraging owners from switching providers at the end of their contracts.
The networks have previously suggested that locking devices helps deter theft and fraud.
But the watchdog noted some companies had already abandoned the practice.
Among those companies affected are:
- BT and its EE mobile division
- Vodafone
- Tesco Mobile
- O2, Sky, Three and Virgin already only sell unlocked handsets.
“[It] will save people time, money and effort – and help them unlock better deals,” said Ofcom’s connectivity director Selina Chadha.
Vodafone has already responded: “We stand ready to implement these changes when they come into force.”
EE added: “We’ll work with Ofcom to comply with its guidelines.”
