Estimates value worldwide Internet advertising as worth more than $40 billion annually, The Washington Post reported Wednesday.

But programs like Adblock Plus, which has been downloaded 20 million times, could undermine the revenues on which many Web sites depend, the Post reported.

Ad-blocking software is sometimes developed by individuals tired of having to put up with advertising as they surf the Internet.

"Rick752," who refused to divulge his name for fear he would be harassed or bribed away from his blocking project called EasyList, calls himself "a blue-collar guy" who is "playing against some pretty big players."

Google and Yahoo!, among the biggest players in the Internet advertising game, declined to comment when the Post asked about ad-blocking, the report said.

But, Web site producers who rely on advertising revenues have reacted by asking their users not to use the software.

"For every ad that doesn't load, we don't earn the needed revenue we need to generate revenue to pay our mortgages, bills, etc.," Dave Scherer, owner of PWInsider.com, a pro-wrestling Web site, told the Post.