Ofcom has opened an investigation into Three and Vodafone to assess whether they have broken rules on net neutrality.
In a statement today the regulator said it was concerned the two companies had breached European rules which force internet service providers (ISPs) to give users access to all online content equally, without discriminating against particular services or websites.
However, Ofcom said it was looking into Three plans that restrict so-called tethering - connecting other devices, such as laptops, to the internet via a mobile phone - as well as restrictions on the devices in which a sim card can be used, and practices where it intentionally slows down particular categories of traffic, such as peer-to-peer networking.
Meanwhile, Vodafone is being investigated over its "Vodafone Passes", which slow down some categories of traffic, and force customers to use their data allowance for some functions of certain apps, but not for others.
"The ‘open internet’ is the principle of ensuring that web users control what they see and do online – not the broadband provider that connects them to the internet," said Ofcom.
"It’s about people being free to access all lawful internet content equally, without broadband providers discriminating against particular services or websites."
Beginning of the end?
While net neutrality is enshrined in EU law, in December the US repealed its laws, allowing ISPs to charge customers for certain "bundles" of internet usage, giving them faster connections to whatever sites they visit the most.
There has been some suggestion the UK may take another look at net neutrality rules after Brexit.