One of the Czech Republic’s biggest Internet service providers has set the country’s online media on edge by saying it’s considering introducing network-level ad blocking.

O2 Czech Republic, which operates both fixed-line and mobile services, told local media houses it wanted to block ads to improve its customers’ surfing experience. This elicited an angry response from a range of publishers, such as Economia and Czech News Center, which refused to meet with O2 to discuss the matter.

Štěpán Košík, the chairman of media house Mafra, warned that O2’s proposed tampering with online content may one day end up with the censorship of articles. Various publishers have now threatened to simply block O2’s customers from seeing their content at all, if it goes through with its proposal.

Get Data Sheet, Fortune’s technology newsletter.

O2 denied it was going to censor anything, and insisted it would be up the user to decide whether or not to use the feature.

The media companies, of course, currently rely on advertising to fund what they do. O2 CEO Tomáš Budník suggested the introduction of ad blocking could provide an “opportunity” for publishers to start charging for the videos and articles they currently provide for free.

As Fortune reported when mobile carrier group Three announced a similar move a week ago, it is likely that the law will be on the side of the publishers.

The European Union recently agreed on new net neutrality laws which, though they are not yet in force, will ban operators from blocking content—unless it falls into very specific categories such as malware. Sources at the European Commission and the European Parliament have confirmed that network-level ad blocking would therefore be illegal, unless individual EU countries pass specific legislation to exempt it from the bloc-wide net neutrality rules.

It’s one thing for individuals to use ad-blocking tools in their browsers, and quite another for operators to do the same at the network level. This doesn’t just fall foul of the technicalities of net neutrality, but of the core principle itself—generally, while operators claim they want to improve their customers’ experience, they also want to use their gatekeeper role to demand a slice of the action from the big ad networks.​

For more on ad blocking, watch:

It’s not yet clear which technology O2 Czech Republic would use to block ads on its network. Shine, the Israeli firm with which Three is working, refused to say whether it was the supplier.

O2 Czech Republic is not, incidentally, part of Telefonica these days (the Spanish carrier group sold its stake to financial group PFF a couple years back). Which is just as well—Ronan Dunne, the CEO of O2 U.K., said in reaction to the Three news that he thought blocking ads at the network level was taking things too far.

This article was updated to include O2’s response to the publishers’ outrage.