A Google spokesman told Times Online: "We cannot tolerate websites trying to manipulate search results as we aim to provide users with relevant and objective search results.

"Google may temporarily or permanently ban any site or site authors that engage in tactics designed to distort their rankings or mislead users in order to preserve the accuracy and quality of our search results."

Google has imposed similar sanctions against other companies in the past. Such actions, which severely lower the internet profile of a site, have become known as "Google death penalties". They see Google reset its search engine so that when a computer user searches for terms linked to a particular company, they do not return direct links to the company’s website.

Google has a policy of re-including sites once it is sure they are no longer violating its guidelines.

The latest move comes at a time when Google is under pressure to revive its reputation for impartiality.

It's recent acquisition of a 5 per cent stake in AOL, the internet portal company, for $1 billion was accompanied by reports that the Google search engine will grant AOL pages a "preferred" status. Analysts said that move risked undermining public trust in the Google brand.

The company has also faced fierce criticism in the West for entering the Chinese market, a move which saw Google accept Beijing's restrictions on free speech.

The BMW action was revealed when Matt Cutts, a blogger claiming to be a Google software engineer, wrote that the company appeared to have used underhand practices to ensure that searches for BMW and "gebrauchtwagen" — used cars — would return the carmaker’s website first.

The practice — known as search engine optimisation — is frequently used by webmasters. But engineers at Google are said to have taken exception to the methods apparently being used by BMW in Germany.

Mr Cutts wrote that the firm had written "gebrauchtwagen" 42 times on its "doorway" page — used to include terms likely to catch surfers seeking used cars. In response, Google has moved to reduce the website’s PageRank to zero, meaning that search results for BMW in Germany no longer yield it as the top selection.

Mr Cutts wrote that the practice was a violation of Google’s guidelines, although it is unclear whether BMW was aware that it was doing anything wrong.

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To have your own say on Google, visit the Times Online technology blog.