The disturbing scale of the personal data harvested and traded by multinationals can be revealed today.
Health details, children's voice recordings and copies of passports can be at risk when customers tick an online consent box.
Analysis by the Mail found that Marriott International, Facebook, Asda, Paypal, BT and Tesco engaged in hidden data harvesting and sharing.
Giant firms can use personal data to build a profile of customers for targeted adverts or to pass to other organisations.
- Pregnant women's due dates being farmed out by Asda to mystery third-party companies for marketing;
- Children's voices recorded on the YouTube Kids app being used by Google to promote other apps;
- Passport copies given to PayPal for account verification purposes being shared with Microsoft for facial recognition products;
- Health details, ethnic origin and political views of Facebook users being used by the social network for targeted advertising;
- Viewers of BT television being profiled for advertisers according to profiles of their television watching and telephone call records.
Emails detailing how Facebook accepted cash in exchange for access to its users' data were published by Parliament last night.
The firm's staff discuss whitelisting companies including AirBnB, Tinder and Netflix – allowing them to retain access to Facebook user data if they placed enough advertising.
Mark Zuckerberg, Facebook's chief executive, wrote in a private email that access to user data could be licensed to advertising buyers.
But he adds: 'If the revenue we get from those doesn't add up to more than the fees you owe us, then you just pay us the fee directly.'
Read more: Is NOTHING private any more? Shocking extent of how big firms harvest your data – from children's voice recordings, passport info and even pregnant mothers' due dates