Google has accused Microsoft of cheating, following an investigation known as the 'Bing Sting'.
Google engineers created 100 nonsensical queries such as "hiybbprqag" and inserted a fake result for each.
Within weeks, the same results began to appear on its rival Microsoft's Bing search engine.
Microsoft denies copying Google and accused Google of conducting "spy-novelesque stunts".
Harry Shum, vice president of Bing, said: "We do not copy Google's search results. We use multiple signals and approaches in ranking search results," he added.
"Opt-in programs like the toolbar help us with clickstream data, one of many input signals we and other search engines use to help rank sites," he added.
