Tech firms have been urged to stop advertising to under-18s in an open letter signed by MPs, academics and children's-rights advocates.

Behavioural advertising not only undermines privacy but puts "susceptible" youngsters under unfair marketing pressure, the letter says.

It is addressed to Google, Amazon, Apple, Facebook and Microsoft.

In a separate move, Google-owned YouTube is accused of unlawfully mining data from five million under-13s in the UK.

European data protection laws forbid the mining of data of young children.

"The fact that ad-tech companies hold 72 million data points on a child by the time they turn 13 shows the extent of disregard for these laws, and the extraordinary surveillance to which children are subjected," the letter reads.

"There is no justification for targeting teenagers with personalised ads any more than there is for targeting 12-year-olds.

"You, the most powerful companies on the internet, have a responsibility to protect your users."

YouTube legal fight

Among the 23 signatories are MP Caroline Lucas and clinical psychologist Dr Elly Hanson. Friends of the Earth is also named on the letter.

It was coordinated by Global Action Plan, which argues that online advertising accelerates consumerism, and adds unnecessary pressure to the planet.

All the firms involved have been asked to comment but none has yet responded.

Separately, privacy advocate Duncan McCann is suing Google on behalf of five million British children, claiming it broke privacy laws by tracking children online, in breach of both the UK and European data protection laws.

The case, lodged with the UK High Court in July, will be strongly contested by YouTube which will argue its platform is not for children aged under 13.

Mr McCann, who has three children under that age, believes damages of between £100 and £500 could be payable to children who are found to have had their data breached.