Google has accused Microsoft of using Google's search results to improve the performance of Bing, Microsoft's own search engine.
The internet firm noticed "surprising similarities" between the top results of the rival search engines, according to the New York Times.
Google suspects Microsoft is using Internet Explorer 8 and the Bing toolbar to send Microsoft data on how people use Google.
To confirm the suspicions, Google set up a sting operation by matching 100 random text queries with random results.
The company then asked 20 of its engineers to install Microsoft's IE 8 with the Bing toolbar, search for the rigged words, and click on the made-up results.
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Google claims Bing soon started pointing users to the nonsensical search results for some of the random text queries set up for the sting.
Danny Sullivan, head of the blog Search Engine Land and industry analyst, said the results strongly suggest that Bing was copying Google's results.
Harry Shum, corporate vice-president of Bing, called Google's sting "a creative tactic" by a competitor, but admitted Bing watches what people click when they visit Google and other sites.
Bing uses over 1,000 different signals and features in its ranking algorithm, he wrote in a blog post.
"A small piece of that is clickstream data we get from some of our customers, who opt-in to sharing anonymous data as they navigate the web in order to help us improve the experience for all users," he wrote.
Google has challenged Microsoft to reveal how much data it had collected from Google users and how this was being used for Bing.