Apple’s chief executive has called for regulation to tackle the “shadow economy” of data brokers – intermediaries who trade in the personal information of largely unsuspecting consumers – as the company continues its push to be seen as supportive of privacy.

Tim Cook, in an op-ed for Time Magazine published on Thursday, said: “One of the biggest challenges in protecting privacy is that many of the violations are invisible. For example, you might have bought a product from an online retailer – something most of us have done.

“But what the retailer doesn’t tell you is that it then turned around and sold or transferred information about your purchase to a ‘data broker’ – a company that exists purely to collect your information, package it and sell it to yet another buyer.

“The trail disappears before you even know there is a trail. Right now, all of these secondary markets for your information exist in a shadow economy that’s largely unchecked – out of sight of consumers, regulators and lawmakers.”

Cook has called for “comprehensive federal privacy legislation” in the US to establish a registry of data brokers, which would enable consumers to check what data of theirs was being sold, and to then easily remove from that market anything they wanted, “on demand, freely, easily and online, once and for all”.

The largest data brokers, companies such as Acxiom, Experian, Oracle and Criteo, have already come under scrutiny in Europe, after the campaign group Privacy International filed a series of complaints in November asking regulators to investigate whether the basis of their businesses fell foul of GDPR, the European privacy regulation.

Speaking at the time, Ailidh Callander, Privacy International’s legal officer, said: “The data broker and ad-tech industries are premised on exploiting people’s data.

“Most people have likely never heard of these companies, and yet they are amassing as much data about us as they can and building intricate profiles about our lives. GDPR sets clear limits on the abuse of personal data.”

Over the past year, Apple has been ever more aggressive in turning its approach to user privacy into a competitive advantage against rivals such as Facebook and Google. Earlier this month, the company was reported to have hired a former Facebook worker-turned-privacy activist, Sandy Parakilas, to work on its privacy team.

Apple’s latest marketing effort was also paraded at the Las Vegas-based Consumer Electronics Show. While Google had invested millions in pushing its AI-powered smart assistant, Apple ran a huge advert bearing the slogan: “What happens on your iPhone, stays on your iPhone.”