Google has weathered criticism in the past that it has copied some features of Microsoft’s Bing search engine, like background images. Now it has turned the tables, contending that Bing copies something much more important: search results.

Danny Sullivan, head of the blog Search Engine Land, wrote Tuesday about a “sting operation” by Google that the company says proves that Bing watches Google’s search results to improve its own.

Google says it suspects Microsoft is doing this by using Internet Explorer 8 and the Bing toolbar, both of which send user data to Microsoft, to watch how people use Google.

In a company blog post, Microsoft did not deny that. But Harry Shum, corporate vice president of Bing, called Google’s sting “a spy-novelesque stunt” and “a creative tactic by a competitor.”

Google grew suspicious when it noticed a few odd things about Bing’s search results, such as surprising similarities between the top results in the two search engines — including results that Google considered to be mistakes, Amit Singhal, who oversees Google’s search ranking algorithm, wrote in a company blog post.

So Google went into detective mode.

It invented about 100 gibberish search queries, like “hiybbprqag,” and matched them with results that had nothing to do with the query, like a theater seating chart. Mr. Singhal likened these queries to “the search engine equivalent of marked bills in a bank.”

Then it asked 20 of its engineers to install Microsoft’s Internet Explorer 8 with the Bing toolbar, search for the rigged words and click on the made-up results. Sure enough, Bing soon started pointing people to the nonsensical search results for seven to nine of the 100 queries.

The result? “Some Bing results increasingly look like an incomplete, stale version of Google results — a cheap imitation,” Mr. Singhal wrote.

Mr. Sullivan, a long-time search industry analyst, came to the same conclusion. “It strongly suggests that Bing was copying Google’s results, by watching what some people do at Google via Internet Explorer,” he wrote in his comprehensive piece.

Mr. Shum wrote that Bing uses more than 1,000 signals to determine search results — including “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.” Translation: Bing watches what people click when they visit Google and other sites.

The tussle is the latest in an ongoing battle between Google and Microsoft on many fronts. In search, Google has prided itself most on the relevance of its search results — which no doubt contributes to its 67 percent search market share. So it is particularly sensitive for Google that Bing, in some cases, mimics its top results.

Mr. Sullivan wryly noted that when Bing started in 2009, people joked that it stood for “Because It’s Not Google,” and that now it may as well stand for “Bing Is Now Google.”