Next time you perform a routine web search, look carefully at your top 10 results. Most users still naively believe their search engine of choice has simply trawled the web for the best possible matches.

But the chances are that several of the sites listed high in the rankings will have paid for the privilege, either through a straight pay-for-placement listing, or, more subtly, through use of a search engine optimisation (SEO) specialist.

While still a relatively new stock-in-trade, the web's increasing commercial clout, combined with belated recognition of the search engine as the online world's most critical tool, are together making SEO one of the fastest growing new niches in the internet economy.

A glance at the figures explains why. A medium-sized company might spend tens of thousands of pounds developing and maintaining a professionally-designed site filled with useful content and maybe even online sales capability. But studies reveal that the vast majority of commercial sites actually damage their own chances of being found by the Big Six search engines - Yahoo, Google, MSN, AOL, Lycos and AltaVista - through a simple lack of understanding of what the search engine "spiders" that trawl the web are looking for.

Popular animated gizmos based on products like JavaScript or Flash, for example, are to all intents and purposes invisible to search engine crawlers. So are Active Server Pages, JavaServer pages, elements contained within frames, and pages that enforce acceptance of cookies - the hidden files sent by web servers to individual PCs that are used by websites to identify users.

Yet a study recently commissioned by iProspect, a US-based SEO pioneer, shows that 97 per cent of websites belonging to Fortune 100 companies routinely employ one or more of these technologies. Even more surprising, 50 per cent of the keywords nominated by the companies as online "signposts" to their sites failed to win that company a significant ranking on the major engines.

With around 80 per cent of the web's estimated 430m users routinely relying on one of the top search engines to find the information they want, that is a lot of lost leads for any company intent on taking its web presence seriously. "An extraordinary number of multi-million dollar sites are literally dead on arrival for the major engines," says Frederick Marckini, iProspect's chief executive and the author of three best-selling books on search engine positioning.

"To effectively optimise your site, it is important to understand that search engines essentially do two basic things: index text and follow links. If your site doesn't contain these, you've just made yourself invisible," he says.

Virtually alone in the field when it was founded back in 1996, iProspect has built a solid business raising the online profile of clients such as 3M, Sharp Electronics and John Deere. Now, the company is being joined by a growing band of SEO specialists around the world eager to exploit what many believe to be not only one of the internet's best opportunities, but a must-have service for just about any company.

In Europe, the market already boasts a handful of dedicated SEO professionals, including Sticky Eyes, NetBooster, Search Engineers and Web Gravity, along with an increasing number of tech-savvy PR and advertising agencies, such as Brodeur Worldwide, which has recently added SEO to its list of client services.

All agree that the key to achieving high search rankings lies in the effective use of meta-data (invisible tags contained within a site's HTML script that provide information on content), along with a thorough understanding of the real-life words and phrases surfers actually use to find information, and simplicity of coding and design.

Surprisingly, perhaps, for such a tech-centric environmen