Google (GOOG) unveiled major changes to its shopping business on Thursday that are likely to prove controversial in the e-commerce world.

Starting in the fall, product search results for users in the United States will be influenced by how much retailers and advertisers pay, a company executive said. In the past, product search results were based mainly on relevance, and the program was free.

The change could affect some results that users see when they run a standard search, because it could affect what is shown in boxes marked "Shopping," which will also carry the word "Sponsored," on a standard results page. But the bigger effect will be on results that users see when they click on "Google Shopping" -- the name the company is giving to the service previously called "Google Product Search."

"We are starting to transition Google Product Search in the U.S. to a purely commercial model," said Sameer Samat, vice president of product management at Google Shopping. "This will give merchants greater control over where their products appear on Google Shopping."

Google has been in the product listing and search business for about a decade. During that time, it has provided merchants with free access to shoppers. The company made money by running paid product search ads along with unpaid

product listings, according to Eric Best, CEO of Mercent, which helps retailers sell through Google and other e-commerce websites such as Amazon and eBay (EBAY).

"Today, that model goes away," Best said. "It's a very big deal."

The changes may ultimately help Google extract more revenue and profit margin from its retail advertisers, which account for up to 40 percent of Google's advertising base, Best said.

Calling the change a significant move for Google, analyst Danny Sullivan wrote on the industry blog Search Engine Land that Google has previously said it wouldn't adopt the practice of "paid inclusion" because it wanted to assure users of unbiased results.

Sullivan noted that Google now says that having a commercial relationship with merchants will allow it to provide more accurate results. He also noted that Google says the new results will be marked "Sponsored" to differentiate them from standard search results.

For retailers, there are upsides and downsides, said Best.

"The downside is that retailers are going to have to pay for performance when it comes to e-commerce traffic and revenue driven by or through Google," Best said. "The free traffic is disappearing."

The changes may be controversial in the Internet community because Google's search results have traditionally not been influenced by money, Best said.

"Pay-for-placement to some degree is an alternative to purely organic relevancy results," he said. "The fact that shopping results will be more closely tied to bid-for-placement will not sit well with all advertisers."

The new program will help retailers make their products more visible to shoppers searching on Google. The old system was difficult for Google to police because retailers could list a lot of products for free. If they have to pay, it may reduce clutter, Best explained.

"Having a commercial relationship with merchants will encourage them to keep their product information fresh and up to date," Google's Samat wrote in a blog Thursday. "Higher quality data -- whether it's accurate prices, the latest offers or product availability -- should mean better shopping results for users, which in turn should create higher quality traffic for merchants."

Mercury News staff writer Brandon Bailey contributed to this report.