Shares of Marvell Technology slumped 9% Monday after a Wall Street firm argued that the custom chip maker has lost out on a key piece of business with Amazon.com.

Benchmark Equity Research downgraded Marvell shares to Hold from Buy and withdrew its price target in a research note Monday.

"We now have a high degree of conviction that the company has lost both Amazon's [Trainium] 3 and 4 designs to its Taiwanese competitor, Alchip, " wrote analyst Cody Acree, referring to Amazon Web Services' newest artificial-intelligence processors.

The company didn't immediately reply to requests for comment from Barron's.

Benchmark's conclusion is controversial, the firm conceded. Marvell shares jumped 11% last week after the company forecast better-than-expected growth for its data-center segment over the next two fiscal years. That guidance helped assuage concerns about whether Marvell could win long-term deals with hyperscalers like Amazon and Microsoft.

J.P. Morgan analyst Harlan Sur concluded from Marvell's outlook that the company had won the Trainium 4 program. The firm reiterated an Overweight rating and raised its price target to $130 from $120 in a research note.

Benchmark disagreed. While Acree believes "the company is being forthright in its guidance for increasing annual Amazon revenue," he thinks the upbeat numbers come from continued volume on programs like Trainium 2 -- not new wins. Benchmark based this conviction on recent industry meetings across Silicon Valley.

"Accordingly, with Marvell's shares performing exceptionally well following its earnings report, we recommend investors take near-term profits on an overly optimistic misread of the company's recent signals toward Amazon stability," Acree wrote.