Three and Vodafone Ofcom are facing an investigation into whether they breached EU net neutrality rules.
Three is being accused of breaching the rules by restricting tethering - connecting a PC to the internet via a mobile connection - while Vodafone is being investigated for offering 'Passes' for certain traffic that doesn't count towards data-usage caps.
The EU's net neutrality regulations were only brought-in in 2016. They require that mobile providers treat all internet traffic on their networks equally and don't give preferential treatment to certain sites or services.
According to Ofcom, it's investigating Three for restricting tethering on certain plans; imposing restrictions on the devices in which a SIM can be used; and for traffic management practices such as throttling. The watchdog claims Three is slowing down traffic from particular categories, including video, peer to peer and VPN traffic.
Vodafone is also being probed over alleged 'traffic management practises' in relation to its zero-rated Vodafone Passes, along with its lack of transparency over which apps are 'zero-rated' and won't eat into customers' data allowances.
While Three didn't have much to say on the looming probe, noting vaguely that it would be "working closely with Ofcom to understand their concerns", Vodafone has vehemently denied throttling speeds on Vodafone Passes.
"Our Passes allow customers to access their favourite content without fear of running out of data or attracting out-of-bundle charges. Vodafone does not 'throttle' speeds on Vodafone Passes, either in the UK or while customers are roaming, a Voda spokesperson said.
"The Video Pass is optimised so that all of our customers have a high-quality experience when streaming content on the network. Optimising means making the bandwidth available that enables videos to be delivered in a faster, more efficient way, while still providing the best smartphone viewing experience, and without compromising the experience of other customers who do not use a Vodafone Pass."
Ofcom said it will publish the findings of its investigation in June.
Traffic throttling is nothing new. Last year, O2 admitted that it was deliberately slowing down customers' mobile internet speeds when they were roaming within the EU.
"Data roaming surpassed all expectations we therefore have put temporary measures in place to protect the service experience for customers roaming in our Europe Zone," O2 said at the time. "These firewalls are temporary and we are working to have these controls removed within the coming weeks."
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