A woman has won an unprecedented victory over Meta and Google, which she blamed for her childhood addiction to social media.

In a test case, a US jury found the technology giants deliberately built platforms that were addictive.

The landmark trial is being likened to a “big tobacco” moment that could open the floodgates for lawsuits.

The companies have been ordered to pay $US3 million ($4.3 million) in damages to the woman who said their products harmed her mental health as a child.

The woman, now aged 20 and referred to as Kaley, said she started watching YouTube at age six and was addicted to Instagram (owned by Meta) by age nine.

Meta will be liable for 70 per cent of the damages and Google for 30 per cent.

“Today’s verdict is a referendum — from a jury, to an ​entire industry — that accountability has arrived,” Kaley’s lead counsel said on Wednesday (US time).

Meta disagreed with the verdict and was “evaluating our legal options”, a company spokesperson said, adding that teen mental health could not be blamed on a single app.

Google planned to appeal, company spokesman José Castañeda said.

The jury in Los Angeles ruled that YouTube and Instagram were ‌negligent in designing their apps ⁠and the companies that owned them failed to warn about their dangers.

The legal victory will influence thousands of ‌similar cases against the tech companies.

The verdict came just a day after a New Mexico jury found Meta had violated the state’s consumer protection law and ordered it to pay $US375 million ($A539 million) in civil penalties. That case had argued the company failed to safeguard its apps from child predators, enabling child sexual exploitation.

Meta has vowed to appeal the verdict.

A separate social media addiction case brought by several US states and school districts against technology companies is expected to go to trial this year in federal court in Oakland, California.

Another state trial is slated to begin in Los Angeles in July, said Matthew Bergman, one of the lawyers leading the cases for the plaintiffs. It will involve Instagram, YouTube, TikTok and Snapchat.

Punitive damages for the companies will be decided next.

The jury may consider whether Google or Meta’s products caused the plaintiff physical harm or whether the companies disregarded the health of other users, Judge Carolyn Kuhl said in court.

Shares of Meta were up 1.0 per cent and Alphabet shares were up 0.2 per cent, little changed after the verdict.

The plaintiffs in the Los Angeles proceeding focused on platform design rather than content, making it harder for the companies to avert liability.

Snap and TikTok ⁠were also defendants in the trial. Both settled with the plaintiff before it ‌began.

Large technology companies in the US have faced mounting criticism in the past decade over child and teen safety. The debate has now ‌shifted to courts and state governments.

The US Congress has declined to pass comprehensive legislation regulating social media.

At least 20 US states enacted laws ​last year on social media use and children, according to the nonpartisan National Conference of State Legislatures, which tracks state laws.

The legislation includes bills that regulate the use of mobile phones in schools and require users to verify their ages to open a social ​media ​account.

NetChoice, a trade association backed by tech companies such ​as Meta and Google, is seeking to invalidate age verification requirements in ‌court.

Last month, Meta boss Mark Zuckerberg repeatedly said the company banned children under 13 from Facebook and Instagram.

The appearance was the billionaire Facebook founder’s first time testifying in court on Instagram’s effect on the mental health of young users.

Mark Lanier, the woman’s lawyer, confronted Zuckerberg with internal Meta documents.

“If we want to win big with teens, we must bring them in as tweens,” read one internal Instagram presentation from 2018.

“Yet you say that we would never do that,” Lanier said.

Zuckerberg said Lanier was “mischaracterising what I am saying”.

He said Meta had “had ‌different conversations over time to try to ⁠build different versions of services that kids can safely use”.

For example, Zuckerberg said Meta discussed creating a version of Instagram for children ​under 13, but ultimately never did.

-with AAP