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A European Union agency has said that mobile network's Three's plans to offer ad-blockers would violate net neutrality.

Net neutrality is a founding principle of the internet that dictates all web traffic should be treated equally.

The Body of European Regulators for Electronic Communications (Berec) said ads should not be blocked by networks.

Three has already trialled blockers for banners and pop-up adverts, and intends to press on with the scheme.

"Berec has provided guidance to the EU's national regulators about their interpretation of the EU's net neutrality legislation - it is not an final decision and the final decision rests with [the UK's regulator] Ofcom," the network said.

"We have not launched our mobile advertising product yet and will continue to keep Ofcom informed of our plans."

Berec published its guidelines on 30 August, clarifying how rules issued earlier in the year should be applied.

The body - which represents all the EU's communications regulators - said that telecoms companies "should not block, slow down, alter, restrict, interfere with, degrade or discriminate advertising when providing an IAS (internet access service)".

Earlier this year, the Internet Advertising Bureau (IAB) said the blocking of adverts could cause consumers to pay for content they'd currently get free.

Three's proposed blockers could prevent 95% of adverts, with its 8.8 million customers given the choice whether to activate the service.

In February, the company said such measures would , rather than do away with adverts altogether.

"Our objective... is not to eliminate mobile advertising, which is often interesting and beneficial to our customers, but to give customers more control, choice and greater transparency over what they receive," the firm said in a statement which announced its partnership with ad-blocker company Shine.

"Customers should not have to pay data charges because of advertising, mobile ads should not access handset data without explicit consent, and phone owners should only see advertising that is relevant and interesting to them rather than obtrusive and untargeted information."

Ad-blockers have become increasingly popular in recent years, but are also viewed as a threat to businesses relying on online advertising revenue.

Earlier this month, Facebook announced it would force desktop users .

Users will be given the choice over the adverts they are shown by selecting the companies and brands they prefer.