The new rules were published by the Body of European Regulators for Electronic Communications (Berec) this week, and are intended to act as guidance on implementing net neutrality to local watchdogs such as Ofcom in the UK.
Net neutrality is the idea that all internet users should have equal access to content online, including advertising.
A clause in Berec's guidelines states that internet service providers and operators "should not block, slow down, alter, restrict, interfere with, degrade or discriminate advertising when providing [a service]."
There are limited exceptions to the rule, meaning operators could block content to comply with the law, protect the network from security breaches or mitigate against heavy traffic on the network.
Ad-blockers installed voluntarily by the user will not breach net neutrality rules.
Legal experts suggest that this spells trouble for operators and ISPs such as Three, which block ads at network level.
"[It] is difficult to see how providers could rely upon the limited exceptions to implement permanent advertising blocking measures within their networks," says Angus Finnegan, head of communications at Taylor Wessing.
"However this doesn't necessarily mean that advertising blocking capability is unlawful and much will depend on the manner in which the blocking is implemented."
How Three might argue the toss
It is this latter point where Three, which trialled a blunt ad-blocking service in partnership with Shine earlier this year, is expected to argue that its service is not in breach of net neutrality rules.
Sources with knowledge of the matter pointed out that Three's ad-blocking trial was opt-in, meaning consumers were given the choice to block ads or not. That means it is not blocking content by default, and so is not in breach of the rules.
The sources added that Three is evolving its final product and is in discussions with industry trade bodies, advertisers, publishers and Ofcom on what the service will look like. They added that the operator was taking care that its final product won't breach the rules.
The sources suggested that Three will move away from the ad-blocking narrative, and more into determining what "good" advertising looks like to the consumer. The final version of its service with Shine will entail better targeting and relevance, and will show ads that adhere to industry standards.
The sources added that the Berec rules are guidance rather than law, and that it's up to Ofcom to determine what consitutes a breach of net neutrality in the UK.
Steve Chester, director of data and industry programmes at the IAB, said: "It's up to Ofcom to determine, knowing Three's approach to the market, to adjudicate whether it's in breach.
"It comes to the minutiae like how they implement, how consumers opt in, do consumers have the right information, and do they understand the implications of that information."
Ofcom wouldn't comment on specific cases, but said it would monitor for breaches and investigate any complaints on net neutrality.
A spokesman said: "Ofcom will monitor compliance with the new rules, and look into any complaints received.
"We will consider any potential breaches as they arise in accordance with our interpretation of the Regulation, and drawing upon the Berec Guidelines to inform our approach."
Three declined to comment.