The Duke of Sussex has thanked bereaved families mounting a legal challenge against two of the world's biggest tech companies for "telling your stories over and over again".

Prince Harry made the remarks in an at-times emotional address to parents gathered after the start of a landmark trial in California that will examine whether Instagram and YouTube damage young people's mental health.

The families are alleging that the online platforms are addictive by design. Lawyers for YouTube and Meta, which owns Instagram, reject the allegations.

Prince Harry expressed hope for "truth, justice and accountability" in footage shared exclusively with BBC Breakfast.

"We've said time and time again that this is a David versus Goliath situation," the prince told the bereaved families, who had gathered on Wednesday evening following the day's court proceedings.

"I've been in some similar situations myself - vastly different - but when you're sitting in court and if you have that feeling of just overwhelming emotion, because you can't believe that the people on the other side are saying what they're saying - that by the very nature of defending what they're defending, the lies that they are stating, is devaluing life, is devaluing your children's lives - if that brings stuff up for you, it is totally normal," he said.

"None of you should be here," Harry said with an audible warble to his voice.

"Thank you for doing everything that you've done. Thank you for telling your stories over and over again. Truth, justice and accountability: those are the three things that will come from this."

She told BBC Breakfast that she was among the roughly 50 bereaved parents who were addressed by the prince on Wednesday.

"We can't make a difference for our children, but we can make a difference for other people's children," she said.

Roome told the programme the trial was "the first of its kind" and that "the world is waking up" to the danger that "harmful" social media content can pose to young people.

The case would force the tech giants to take "accountability", she said, adding that she thought "these companies are a law unto their own".

The BBC has approached YouTube and Meta for comment.

Meta has said in the past that it wants to keep young people safe online. Over the last two years, it has rolled out certain features for "teen accounts" on Instagram, including content filtering for users under 16 years of age. YouTube also has various features to limit content for younger users.

The outcome of the case brought in California may also affect separate cases brought by attorneys for dozens of US states seeking tougher restrictions on Meta platforms.