BP is being accused of trying to manipulate the search results on sites like Google and Yahoo, as it attempts to salvage its battered image following the oil slick in the Gulf of Mexico.

The company is purchasing terms such as "oil spill", "Deepwater Horizon" and "Gulf of Mexico", so that when a user types these words into the search engines, the results prominently feature a "sponsored link" to BP's official page on its response to the spill.

Critics have described BP's move as unethical. Maureen Mackey, a writer on the Fiscal Times, an online news site, said: "What it effectively does is that it bumps down other legitimate news and opinion pieces that are addressing the spill... and [BP are] paying big money for that."

The criticism comes as President Obama expressed unease at the amount of money the company was spending to counter the negative attention the company has received following the oil spill.

BP has confirmed that its digital teams based in Houston and London, together with the company's marketing executives, are currently engaged in buying search terms.

The company sought to downplay the strategy, saying that it was aimed at helping those most affected by the spill, by providing accurate information on the correct forms to fill in and key people to contact.

When a user types any term into Google, the search engine returns the most relevant internet links relating to that term. In addition, companies can bid against each other, so that their advert also appears in the search results. These "sponsored links" are clearly distinguished and can appear above or alongside normal search results.

Groups can bid pennies or thousands of pounds for a search term, but the highest bid does not necessarily win. Google demands that adverts are "relevant" - for example, that the link is proven useful as many people have clicked on it.

BP have not revealed how much buying search terms such as "oil slick" has cost the firm. Companies are charged "per click", meaning the more people click on the adverts, the more it will cost the firm. The New York marketing analyst Scott Slatin, who specializes in search engines, has estimated that BP is paying search engines over $10,000 a day to "own the top positions".

Other analysts say that the move is a legitimate tactic that has been used successfully by other organisations in crises.

As the importance of the internet has grown, companies have increasingly tried to control their public image through buying advertising on search engines. Political parties across the world, include the Conservative Party during this years general election, have bought key search terms to ensure their messages are at the top of search engine results.