The White House has declined to comment about the image
Google has apologised over a racially offensive picture of Michelle Obama that appeared when users searched for images of the US first lady.
The image came top of the Google Images results for "Michelle Obama".
Google placed a notice over the picture titled "Offensive Search Results", saying: "Sometimes our search results can be offensive. We agree."
Later on Wednesday the image dropped from top image results, though the BBC understands Google did not remove it.
Instead, the image appeared to have been removed from the site that originally published it, and was therefore no longer appearing prominently in Google searches.
The White House has declined to comment.
Users who click on the advertisement above the image were directed to a statement from Google, which explained that its results "can include disturbing content, even from innocuous queries".
Martin Asser, BBC Search Engine specialist
Freedom of access to the internet means much offensive material resides there, but ordinary users are seldom exposed to it in daily online activity.
Occasionally, however, the world's most popular search engine, Google, highlights such material, which is what has happened with a mock-up photo of Michelle Obama.
The cause is Google's system of algorithmic analysis, which, without human intervention, ranks sites according to things like the number of links pointing there and the amount of activity at the site.
The shocking doctored image of Mrs Obama must have immediately created a flurry of interest - mainly negative - and that sent the image shooting up Google's rankings.
"We apologise if you've had an upsetting experience using Google," the company said.
Google says a website's ranking in its search results relies heavily on computer algorithms, using thousands of factors to calculate a page's relevance to a given query.
But the search engine says it does not remove images simply because it receives complaints.
"Google views the integrity of our search results as an extremely important priority," it said.
"Accordingly, we do not remove a page from our search results simply because its content is unpopular or because we receive complaints concerning it."
However, the California-based web giant says it will take down certain images, if required by law to do so.
A spokesman for Google, Scott Rubin, would not give details on how the image - which has sparked fury in the blogosphere - ended up as top result for the wife of President Barack Obama.
The picture first surfaced earlier this month, when it was removed because the site hosting it violated Google guidelines by spreading so-called malware - malicious software designed to infiltrate other computers.
But the image then reappeared on another site, apparently untainted by malware, meaning Google was bound by its own rules not to meddle with the search, according to technology analysts.
David Vise, a Pulitzer prize-winning journalist and author of The Google Story, told the BBC the search engine's results get to the top based on popularity, not because of any ranking system by people.
He added: "If Google got a call from the White House telling them it's against the law to have an offensive image of this kind which portrays the first lady in a racist manner as a monkey or an ape, then they would be obliged to take it down and I'm sure they would do so immediately."
But he said it would be a "very slippery slope" if Google were to try to police the limits of free speech.
"Once you begin to block images, who is to say. It's like the Supreme Court of the United States once said, 'what is pornography?' Well we can't define it, but we know it when we see it."