The move would have a big impact on platforms such as YouTube and Facebook, and civil liberties activists are worried that the proposals would lead to greater online surveillance and censorship.

The proposals unveiled Thursday by the European Commission—the EU’s executive body, which comes up with new laws—include the idea that online platforms should build “automatic detection technologies” in order to prevent the reappearance of content that has already been flagged as illegal.

They also call for “voluntary proactive measures to detect and proactively remove illegal content.”

For now, these are proposals for new guidelines that online platforms should follow evenly across the EU—countries such as Germany already have laws forcing platforms to do more to tackle illegal content. However, the proposals come with the threat of EU-wide legislation if the platforms don’t abide by the guidelines voluntarily.

“We cannot accept a digital Wild West, and we must act,” said EU justice commissioner Vera Jourova. “The code of conduct I agreed with Facebook, Twitter, Google and Microsoft shows that a self-regulatory approach can serve as a good example and can lead to results. However, if the tech companies don’t deliver, we will do it.”

The move, part of a wider package of anti-terrorism measures, was anticipated and critics were quick to hit back.

“Many EU member states are urging big tech companies to take more proactive, automated measures to take content down. This is extremely dangerous,” said Marietje Schaake, a Dutch liberal member of the European Parliament. “The Commission should be pushing back against the trend, not embracing it. There can be no room for upload filters or ex-ante censorship in the EU.”

Schaake added that such measures could inspire new laws in repressive countries such as China, Russia, and Turkey.

EuroISPA, the association of European internet service providers, also warned against “privatized enforcement [that] undermines due process and natural justice.”

However, digital economy commissioner Mariya Gabriel insisted that the current situation was unsustainable. “In more than 28% of cases, it takes more than one week for online platforms to take down illegal content. Today we provide a clear signal to platforms to act more responsibly,” she said.

EU legislators are currently also considering copyright proposals that would also force platform providers to monitor everything their users upload, in order to filter out copyright-infringing files.

In both cases, there may be clashes with existing legislation—the e-Commerce Directive—that says web firms don’t need to proactively monitor what goes onto their platforms. The Commission said Thursday that it did not see a conflict, though.