A top EU court has ruled Google must amend some search results at the request of ordinary people in a test of the so-called "right to be forgotten".
The European Union Court of Justice said links to "irrelevant" and outdated data should be erased on request.
The case was brought by a Spanish man who complained that an auction notice of his repossessed home on Google's search results infringed his privacy.
Google said the ruling was "disappointing".
"We now need to take time to analyse the implications," a spokesperson added.
The search engine says it does not control data, it only offers links to information freely available on the internet.
It has previously said forcing it to remove data amounts to censorship.
The EU Justice Commissioner, Viviane Reding, welcomed the court's decision in a post on Facebook, saying it was a "clear victory for the protection of personal data of Europeans".
"The ruling confirms the need to bring today's data protection rules from the "digital stone age" into today's modern computing world," she said.
The European Commission proposed a law giving users the "right to be forgotten" in 2012.
It would require search engines to edit some searches to make them compliant with the European directive on the protection of personal data.
In its judgement on Tuesday, the court in Luxembourg said people had the right to request information be removed if it appeared to be "inadequate, irrelevant or no longer relevant".
A right to be forgotten?
- In 2012, the European Commission published plans for a "right to be forgotten" law, allowing people to request that data about themselves to be deleted
- Online service providers would have to comply unless they had "legitimate" reason to do otherwise
- The plans are part of a wide-ranging overhaul of the commission's 1995 Data Protection Directive
- UK's Ministry of Justice claims that the law "raises unrealistic and unfair expectations"
- Some tech firms have expressed concern about the reach of the bill
BBC technology correspondent Rory Cellan-Jones says the ruling has huge consequences for anyone who publishes material online about individuals.
It appears to say that anyone who does not like an old story about them can ask for it to be wiped away, he adds.
The judgement stresses that the rights of the individual are paramount when it comes to their control over their personal data, although there is a public interest defence when it comes to people in public life.
'No legal oversight'
The ruling came after Mario Costeja Gonzalez complained that a search of his name in Google brought up newspaper articles from 16 years ago about a sale of property to recover money he owed.
He said the matter had been resolved and should no longer be linked to him.
Campaign group Index on Censorship condemned the decision, saying it "violates the fundamental principles of freedom of expression".
"It allows individuals to complain to search engines about information they do not like with no legal oversight," it said.
"This is akin to marching into a library and forcing it to pulp books."
Mr Gonzalez's case is one of scores of similar cases in Spain whose complainants want Google to delete their personal information from their search results.
The court said people should address any request for data to be removed to the operator of the search engine, which must then examine its merits.