The letter was sent on behalf of Jean-Claude Juncker, the president of the European Commission. REUTERS/Yves Herman In the battle against ad blocking, many publishers have begun preventing readers from viewing content while they have an ad blocker switched on.

However, a letter purporting to be from the European Commission suggests that these publishers could be breaking European law.

The letter was sent to Alexander Hanff, a privacy campaigner. It confirms that the publishers' mechanism to detect ad blockers requires access to people's personal data. For websites to legally access your personal data, you must have given permission, according to The Register, where we first spotted the story.

Here's the letter:

It follows from this that people must give consent before publishers detect whether or not they are using an ad blocker. Most publishers that are using ad-blocker detection software do not appear to be doing this. Therefore, according to Hanff, they are breaking the law.

Hanff says he will now launch legal challenges against websites using anti-ad blockers without asking permission. Hanff, CEO of Think Privacy, has also said that he will release a "Name and Shame" list of those sites he believes to be acting illegally.

If publishers are forced to ask people permission to access their data before deploying ad blocker detection, they must either (a) hide content from browsers until they have given permission to release personal data and risk putting them off viewing the site entirely or (b) show content before the permission to access personal data has be given, greatly reducing any incentive for browsers share this data.

There are close to 200 million people using ad blocking software each month, according to data from a recent BI Intelligence report.