Some of the world's most popular websites have been penalised by Google in what may be the first round of a wider battle about how much power the search firm wields over the internet.

The Washington Post, Engadget - the popular gadget blog - and Forbes, the business publication, are among the sites to have had their Google rankings lowered after they were found to be embedding 'paid for' links on their pages - a practice the search company is keen to stamp out.

Google said it had lowered several sites' PageRank - a mark it gives them out of 10 which is supposed to reflect how useful they are - where they had been found to be receiving money in return for linking to sites so that the latter could in turn perform better in search queries.

The 'PageRank' mark does not relate directly to how a site performs performs in Google search results, but the qualities it reflects - a site's reputation, as well as the number of other sites that link to it - are similar to those which Google's main engine uses to rank sites on the web.

Google did not accuse the affected sites of taking 'cash for links', but said the recent update to its rankings system, as a result of which the Washington Post found it had been dropped from a 7 ro a 5 (out of 10), was "primarily regarding PageRank selling and the forward links of sites."

"Paid links that pass PageRank would affect our opinion of a site," Matt Cutts, the chief 'search quality' officer at Google, said. "Going forward, I expect that Google will be looking at additional sites that appear to be buying or selling PageRank.

Bloggers rounded on Google after it quietly implemented the changes, which also affected TimesOnline, saying there was no reason they should be penalised for selling links, particularly if the sponsorship was clearly displayed.

"Why should bloggers be penalized if they provide quality content and sell links? Why slight them?" Barry Schwartz of the Search Engine Roundtable blog, was quoted as saying.

Loren Baker, the editor of Search Engine Journal, said that it wasn't only sites with paid-for links that were affected, and that sites which did a lot of cross-linking - a common practice among bloggers - also appeared to have been targeted.

Google's willingness to adjust sites' rankings when it suspects foul play reveals the extent to which it will not tolerate perceived attempts to outwit its engine, which it says ranks websites free of commercial interference.

It also shows the overarching importance of its engine - which handles more than 3 in 4 search queries conducted in the UK - in deciding where the web's traffic is directed.

Links are one of the most valuable commodities on the web. The more links a site has to it, the better it performs in search queries and, in theory, the more traffic it gets, which has meant that a market for trading links has developed.

Although companies are able to 'bid' on keywords so that their sites feature in 'sponsored links' on the right hand side of a search results page, their position in 'organic results' - the ones generated by Google's patented 'algorithm' - should not, at least in principle, be able to be bought.

Rand Fishkin, the chief executive of SEOmoz, a consultancy that helps companies boost their search rankings, said: "The bottom line is that people are able to manipulate Google’s rankings by buying links, and Google has to do something about that."

Google has never penalised sites for selling paid links - which are different to advertisements and often appear in a small box under the word 'sponsored' - but has instead focused on preventing such links from interfering with its engine.

As a result of this week's changes, sites will for the first time risk their own PageRank being reduced if they sell such links.

One explanation for why sites may appear to be embedding paid-for links when in fact they are not is that Google's crawler is mistaking classified ads - which should not figure in its engine's calulations - for such links.

Danny Sullivan, who edits the site Search Engine Watch, said: "Google is not telling people what to do with their sites. It is telling people what to do if they are concerned about doing better in Google. It is saying: don't want to be harmed in Google? Don't sell links."

The Washington Post, Forbes, and Engadget were all unavailable for comment.

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