Yahoo has confirmed that it is preventing some people who use ad blocking software on their browsers from accessing their e-mail.

The move is widely seen as an indication of how the California-based tech giant intends to respond to the growing popularity of ad blocking software among the world's three billion web users, according to the BBC.

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Ad-blocking, a technology that permits web users to block advertisements before they are loaded onto their browsers, is at the center of a controversy that has been brewing for at least four years now.  

Scores of websites and companies that rely on internet advertising for revenues have attempted different strategies to minimize the impact of ad blocking on their businesses, with some taking legal action against the makers of ad blocking software.

Internet forums buzzed with complaints last week as some ad block users found themselves locked out of their Yahoo mailboxes.  

A spokesperson for Yahoo has meanwhile clarified that the restriction is not an official policy of Yahoo Mail just yet, but is part of a routine test used to measure the reaction of a small number of consumers to new products or policies. 

"At Yahoo, we are continually developing and testing new product experiences," the spokesperson told Endgadget. "This is a test we're running for a small number of Yahoo Mail users in the US."  

Ansel Halliburton, an attorney from the internet law firm, Kronenberger Rosenfeld, asserts that Yahoo is well within its rights to demand that users disable any ad blocking software on their browsers when accessing their Yahoo Mail accounts. 

This, the lawyer points out, does not violate any part of Yahoo's legal agreement with consumers. "PR-wise, this seems like a harsh thing to do," Halliburton told the website, Stuff. "But running e-mail infrastructure at that volume takes a lot of resources."   

Much of the heat that now characterizes the issue was first sparked four years ago, when Ad-Block, the most popular ad blocking extension for both Google Chrome and Safari, introduced a so-called whitelist into its operations. 

At first, Ad-Block Plus blocked all advertisements.  In 2011, however, the software makers decided to bundle a whitelist into its product, allowing the ads of an initial 150 websites and companies through its filters.  The whitelist has since become an integral component of Ad Block's downloadable extension and has now expanded to include a total 300 companies. 

To be included in the list, Ad Block requires companies to use only ads that meet its criteria: no animations, do not get in the way of users reading text, and no ads that occupy more than a third of a page's width, among others. 

But Ad Block's fairly strict parameters are not at the root of the controversy.  What has some websites and tech firms up in arms is the fee demanded by the proprietors of the popular browser extension. If a company fails to pay the fee, their ads are blocked, regardless of whether or not they meet the criteria, according to an earlier BBC report 

This has prompted rather strong criticism, with detractors using words like "extortion" and "protection racket" to describe Ad Block's operations. 

TagsYahoo, Ad Block Plus, ad block whitelist, Yahoo Mail, Yahoo Ad Block