Three and Vodafone are being investigated by the communications watchdog over whether they abused net neutrality rules put in place to protect the open internet.
Ofcom on Tuesday said it will scrutinise both companies’ 4G and add-on plans, including whether they have deliberately slowed down certain services, a practice known as “throttling”.
The rules, under the EU Open Internet Access Regulation 2015, state that broadband providers and mobile network operators cannot restrict legal websites, to protect so-called net neutrality. Both face a penalty of up to 10pc of annual turnover.
Three is under investigation for the way it offers price plans that block people from tethering – sharing internet connection from one device with another to get it online.
Ofcom will also assess whether the way Three restricts use of SIM cards in different devices is a breach of the act, and if it is intentionally slowing down particular categories of traffic in the UK and abroad for “roaming”.