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.
Giant firms can use personal data to build a profile of customers for targeted adverts or to pass to other organisations.
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.'
Last week Marriott International announced that hackers had breached its database of 500million guests, with the attackers having 'some combination' of passport numbers, names, addresses and bank card details.
The hotel group also routinely stores the names and ages of its guests' children, room service orders, social media accounts and employer details and shares this across its operations in 150 countries including Venezuela, Gabon and Libya.
The hotel chain faces investigations from the Information Commissioner's Office in the UK, as well as the FBI and five separate American states.
Marriott International is one of a dozen companies investigated by the Mail to assess the full scope of the data taken from customers – details of which are buried within thousands of words of legal jargon.
Last night a spokesman from the ICO said its enforcement team was examining the material we provided. It has the power to fine companies up to £17.7m or 4 per cent of a company's global revenue for data breaches.
Tory MP Damian Collins, who chairs the Commons digital committee, which published the Facebook emails, said: 'This investigation clearly demonstrates that there is a complete data free-for-all where big companies are building up huge banks of data on their customers who, on the whole, are largely unaware of what they are giving away and what happens to it.'
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