Berners-Lee, along with activist academics Lawrence Lessig and Barbara van Schewick, published an open letter to citizens, lawmakers and regulators, declaring there were “four days to save the open Internet in Europe.”

The public has until July 18 to comment on the draft guidelines published by the EU’s telecoms regulators, in which they described how they would apply the new “net neutrality” rules that were agreed by lawmakers last year.

“Network neutrality for hundreds of millions of Europeans is within our grasp,” the letter read. “Securing this is essential to preserve the open Internet as a driver for economic growth and social progress. But the public needs to tell regulators now to strengthen safeguards, and not cave in to telecommunications carriers’ manipulative tactics.”

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The new rules are fuzzy on things like “fast lanes” and so-called zero-rating—where operators don’t charge for the data used by specific services, but do charge for the data used by their rivals.

Zero-rating is not specifically mentioned in the new law, even though it breaks the core net-neutrality principle of treating all online services equally. Regarding “fast lanes,” the new rules allow operators to give priority on their networks to specialized services that apparently require it, such as medical applications and the operators’ Internet TV packages.

The EU regulators’ draft guidelines, published in early June, describe a regulatory regime where it would be difficult, though perhaps not impossible, for operators to abuse these loopholes.

Internet providers would be able to offer zero-rated services, but would have to cut off access to them when users hit their regular data caps. Such deals must also avoid discouraging rival services from entering the market, and the favored services should not be leaders in their market—bad news for operators trying to offer free Facebook access, for example.

The regulators, via their umbrella body, BEREC, also said they would keep a close eye on what operators deem to be specialized services, to make sure they aren’t services that could easily go over the regular Internet.

When Berners-Lee, Lessig and van Schewick warned about carriers “lobbying hard to get regulators to adopt weak guidelines that would benefit their businesses over the public interest,” they were able to point to a recent announcement by Europe’s biggest telecoms operators and infrastructure firms.

Last week, companies such as Deutsche Telekom, Telefonica, Ericsson and Nokia published a manifesto about the deployment of 5G, the next generation of mobile broadband. They strongly implied that their timely investment in the new technology—a major goal for the European Commission—was dependent on being allowed to circumvent strong net neutrality rules, so as to get as much money out of 5G as possible.

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As soon as the new net neutrality rules were agreed last October, Deutsche Telekom also announced—with ill-advised enthusiasm—that it intended to charge startups for prioritization on its network. This could not be more against the spirit of net neutrality, and would almost certainly break the new law.

“We–the ordinary users of the Internet–don’t have expensive lobbyists. But we have millions of people–everyday Europeans, startups, investors, small businesses, activists, NGOs, bloggers, independent artists–who have experienced the power of the open Internet first hand and want to protect it,” Berners-Lee, Lessig and van Schewick wrote.

The authors of the open letter warned not only about fast lanes and zero-rating, but also about carriers’ desire to slow down whole classes of Internet traffic—here, they said operators might use this loophole to slow down all encrypted traffic, even when the network is not congested.

They urged people to comment on the guidelines by filling in an online form. The deadline for doing so is 2pm CEST on Monday, July 18.