Following the Guardian action, Google and YouTube promised to make significant changes to its policies to deal with the problem. A spokesman said: “We have strict guidelines that define where Google ads should appear, and in the vast majority of cases, our policies work as intended, protecting users and advertisers from harmful or inappropriate content. “We accept that we don’t always get it right, and that sometimes, ads appear where they should not. We’re committed to doing better, and will make changes to our policies and brand controls for advertisers.”

David Pemsel, the Guardian’s chief executive, wrote to Google to say that it was “completely unacceptable” for its advertising to be misused in this way.

The use of programmatic trading, which automates the process of buying and selling advertising online , is becoming increasingly controversial amid concerns that it both hurts media revenues and supports extremist material.