For some time, web surfers annoyed by banner and popup ads have been able to use software to screen out the commercial equivalent of 'messages from our sponsor'.

Now online advertisers have their own weapon – the ability to stop surfers that refuse to view such ads from accessing their sites.

German software house MediaBeam has developed antiblocking software, called AdKey, that allows website owners to deny access to users who have installed ad filters.

It’s little surprise that advertisers are fed up. Filtermaker, a Siemens spinoff, claims four million users. AdSubtract users "have blocked over five billion internet advertisements in little over a year, freeing up terabytes of bandwidth," InterMute said in a statement in July.

Websites owners complain that ad filters are choking off their revenue stream and that, in an era of faster Net access, there's no longer a need to filter out ads.

"AdKey detects more or less all the programs . . . [Our server] can detect if someone is trying to get our information without displaying the ads," said Frank Beckert, chief executive of MediaBeam.

AdKey is being used on, MediaBeam's messaging site which gives users access to voice and text messaging services, fax and email via the web and is financed entirely through advertising.

When a surfer who is using ad-filtering software tries to access the site they are given the option to switch off their filter or pay a fee for advertising-free services, said Beckert.

After testing, MediaBeam plans to offer AdKey for Microsoft's Internet Information Services and the open-source Apache web server, Beckert said.