Three has shifted customers to a 4G tariff for free - but might kill off unlimited mobile tethering in the process.

The carrier has upgraded every customer with a 4G-enabled handset to a 4G tariff, making good on a promise to roll out superfast internet for free.

That doesn't mean all 7.9 million Three customers will automatically get superfast internet, since they would need a compatible handset and to live in a 4G area.

Three said 1.7 million of its customers own a 4G-compatible handset, but couldn't say how many were using superfast mobile internet. The carrier is still in the process of rolling out its 4G network, which is currently available in 36 major towns and cities in the UK.

2GB cap on tethering

One side-effect of the upgrade is that Three is reportedly considering killing off free mobile tethering, according to ISP Review.

Currently, the operator offers unlimited data and tethering on its One Plan. No other operator offers unlimited 4G data, even for a fee. Three was also the first to promise superfast internet at no extra cost.

Customers on the One Plan who switch on tethering can turn their phone or tablet into a portable Wi-Fi hotspot, giving them a cheap alternative to fixed-line broadband for no extra cost. Tethering with other operators or on other tariffs usually incurs an extra fee, because of the additional strain on the network.

According to ISP Review, Three will introduce a 2GB monthly cap on tethering for new customers signing up to the One Plan from March. Existing One Plan customers will get unlimited tethering for the duration of their contract.

Three hasn't responded to a request for confirmation, but told ISP Review it couldn't discuss future plans.