The 20-year-old California woman, known as Kaley and identified in court by her initials KGM, testified that she began using YouTube when she was six and Instagram when she was nine.

She alleged that features such as algorithmic content recommendations, beauty filters and push notifications were deliberately designed to be addictive and caused her to develop anxiety, depression, body dysmorphia and suicidal thoughts.

Meta CEO Mark Zuckerberg and Instagram head Adam Mosseri testified during the high-profile proceedings in Los Angeles County Superior Court.

Jurors found that the two companies acted with malice, oppression and fraud in their conduct, ordering them to pay $3 million in compensatory damages and a further $3 million in punitive damages – 70 per cent to be paid by Meta and 30 per cent by YouTube.

The outcome of the case, which both companies intend to appeal, could have an impact thousands of further cases raising similar issues.

TikTok and Snap were also original defendants in the case, but each settled their individual claims on undisclosed terms with the plaintiff prior to trial.

Mark Lanier, founder of The Lanier Law Firm and lead trial counsel for the plaintiff, said: “For years, social media companies have profited from targeting children while concealing the addictive and dangerous design features built into their platforms. Today, we finally have accountability.

“These companies made deliberate choices that prioritised engagement and profit over the well-being of the young people using their products. This verdict sends a clear message to an entire industry that the era of operating without consequence is over.”

Rachel Lanier, managing attorney of The Lanier Law Firm’s Los Angeles office and co-lead trial counsel, added: “Kaley put a human face on what these companies have known for years: that their platforms were engineered to hook young users, and that the children most vulnerable to trauma were the ones they were most effectively reaching.

“This verdict validates what her experience – and the experiences of more than 1,600 other plaintiffs – has shown to be true. It is a turning point not just for this case, but for every family harmed by the calculated choices of these companies.”

A spokesperson for Meta said: “We respectfully disagree with the verdict and will appeal. Teen mental health is profoundly complex and cannot be linked to a single app.

“We will continue to defend ourselves vigorously as every case is different, and we remain confident in our record of protecting teens online.”

A Google spokesperson said: “This case misunderstands YouTube, which is a responsibly built streaming platform, not a social media site.”