Google denies Panda hit on rival
Google has defended recent changes to its search system that reduced the prominence of some popular websites.
One of the worst hit by the "Panda" update was Ciao.co.uk, a Microsoft-owned company that had been leading an EU competition case against Google.
Its web visibility fell by 94% according to analysis by Searchmetrics.
Google's head of search evaluation, Scott Huffman, said it was "almost absurd" to suggest that the results were rigged.
The company regularly changes the algorithms that determine what users see when they search.
Such updates are often done to weed out "content farms" - websites that copy material from other sites in order to get hits.
Where a keyword search may previously have returned their site on Google's first page, afterwards it may be relegated to further down the rankings.
When the update, known as Panda, was rolled out globally on 11 April, Google published a blog post explaining that it was designed to "reduce rankings for low-quality sites".
Shopping and price comparison sites such as Ciao.co.uk sometimes suffer when Google algorithms change because they carry comments and reviews replicated elsewhere on the internet.
However, experts said that it was unusual to see a legitimate website hit as badly as Ciao.
"A 94% drop is astronomical," said Sanjay Shelat, a search engine optimisation (SEO) specialist at Edit Optimisation.
"It is very unusual to take such a hit in an update. That is enough to put a company under."
Ciao.co.uk was involved in initiating an EU investigation into Google in November 2010.
Parent company, Microsoft, claims that Google has used its dominant position to limit rivals' products.
The BBC asked Microsoft if it thought the downgrading of Ciao results was related to the legal action, but Microsoft said it would not be commenting on the situation "at this time".
When questioned by news agency AFP, Google's Scott Huffman said: "If you think of the scale of what we are talking about, it is almost absurd to say we could rig results."
Mr Huffman pointed out that the update had received a very positive response from Google users.
Searchmetrics analysed Google results in response to a range of keywords, both before and after the Panda update.
Alongside Ciao's 94% reduction in visibility, it found that hubpages.com fell by 85% and eHow.co.uk dropped 53%.
A similar analysis by Sistrix found a 81% drop in visibility for Ciao.co.uk, 72% reduction for hubpages.com and an 84% fall for eHow.co.uk.
While a sharp drop in visibility may constitute a crisis for some websites and their search engine optimisation (SEO) engineers, it does not necessarily spell disaster.
Technology news website Electricpig.co.uk was downgraded by 94% by the Panda update, according to Searchmetrics.
Site editor James Holland told BBC News: "We haven't seen an immediate impact.
"Comparing our traffic from Google for that week, we're actually only down 0.5% versus the week before Panda took effect.
"That suggests most of the keywords Searchmetrics are measuring us against weren't being clicked anyway, and our best-performing stuff is still doing the business."