The ruling opens the way for European users to flood the firms with Web takedown requests, adding costs and time to what they already do in content removal.
Many of the companies already deal with compliance for different data laws in various geographies, subjecting requests to shed content to thorough legal analyses before making the information unavailable.
"It's just such a mind-bogglingly impossible decision," said Fred Cate, distinguished professor at the Indiana University Maurer School of Law. "Courts aren't responsible for the practical implications of rulings but this really staggers the imagination."
The EU decision applies to search engines, which means it will affect Google, Microsoft's Bing and Yahoo.
Google and others may now have to consider charging a fee for European users to cover the costs of staff to comb through requests, Cate said. Or they may try to get by with being seen to make a good faith effort to comply, even though any of the 28 European nations governed by the ruling or any zealous local prosecutor could then take a company on for failing to do enough, he said.
"They'd have to hire an army of compliance officers," said Justin Brookman, director of the Center for Democracy and Technology's Project on Consumer Privacy. That may make it difficult for companies to "scalably compete online," he said.
The EU decision illustrates how Europe and the US are diverging on how stringently they approach privacy. That has gathered steam since last year, following Edward Snowden's revelations of the electronic-spying practices of the US National Security Agency.
Google said it was reviewing the EU court's decision, with spokeswoman Leslie Miller calling it a " disappointing ruling for search engines and online publishers in general."
Other Internet companies said they have also started studying the ruling and its implications.
Watch: European Court: Google must yield on info
"Since our founding almost 20 years ago, we've supported an open and free Internet; not one shaded by censorship," said Sarah Meron, a spokeswoman for Sunnyvale, California-based Yahoo. "We're now carefully reviewing the European Court of Justice's decision to assess the impact for our business and for our users."
Jack Evans, a spokesman for Microsoft, declined to comment. Nu Wexler, a spokesman for Twitter, and Genevieve Grdina, a spokeswoman for Facebook, didn't respond to requests for comment.
The EU court said in a statement that it sought a balance between "the legitimate interest of Internet users potentially interested in having access to that information" and privacy rights.
If used in limited cases, "it's probably a positive move that people have their privacy protected," said Danny Sullivan, founding editor of industry website SearchEngineLand.com, in an e-mailed statement. "However, there's a real concern if this turns out to be abused, if done to prevent easy access to legitimate public records."
The EU restrictions are unlikely to help privacy advocates in US courts, Danny O'Brien, a lawyer at the Electronic Frontier Foundation, said in an e-mail. That's because while EU countries rely on a wide-ranging law that covers all personal data processing, the US has a patchwork of privacy laws, he said.
Europeans turn to a privacy regulator to investigate violations, while US privacy violations are enforced through private lawsuits and occasionally the Federal Trade Commission, he said.
Google's Web-search results are based on algorithms, which don't require the interference of humans. Yet the company over time has had to put employees more directly into the work of sorting through content and data as requests to remove online information have multiplied.
The Mountain View, California-based company's legal team now deals with thousands of government requests every year around online data. In the first half of 2013, the number of government requests to remove content from its services rose by more than two thirds to almost 4,000 from the prior six months, according to Google's transparency report at the time.
The requests came from countries including France, Spain and Italy and also from a variety of sources, including court orders and the police, according to Google. The biggest complaints are for defamation, privacy and security.
In 2012, the company received requests from governments including the ones in Turkey and Malaysia to get rid of videos on its YouTube site containing clips of the movie "Innocence of Muslims." The video depicts the Prophet Muhammad as a womaniser and shows a fictional attack by Muslims on a Christian family.
Google doesn't always oblige the inquirers. In Germany, the company complied with 74 per cent of court requests during the first half of 2013, according to Google. In France, it was 58 per cent.
The company also sometimes makes a statement about what it will or won't show in its search results. In 2010, Google pulled its service from China after refusing to censor search results.
Google has also faced calls to remove content in the US. Its YouTube site has long said it will remove videos that infringe copyright issues. In March, Google and Viacom settled Viacom's $1 billion lawsuit claiming YouTube violated copyrights by letting users post video clips from television shows without authorisation.
- Bloomberg
