U.S. antitrust officials sued to block Nvidia Corp.’s proposed $40 billion takeover of Arm Ltd., saying the deal would harm competition in the semiconductor market.

The Federal Trade Commission said in a release Thursday that the acquisition would give Nvidia control over the computing technology and designs that rival firms rely on to develop competing chips.

“The FTC is suing to block the largest semiconductor chip merger in history to prevent a chip conglomerate from stifling the innovation pipeline for next-generation technologies,” FTC Bureau of Competition Director Holly Vedova said in the statement.

Nvidia held most of its gains for the day after the news, and was trading up 2.3% at $321.48 at 3:20 p.m. in New York. That pointed toward another advance for a stock already up 141% this year through Wednesday, blowing past rallies for broad U.S. indexes.

Arm, owned by Softbank Group Corp., is known as the Switzerland of the semiconductor industry. It licenses its technology to hundreds of companies, while competing with none of them.

All major chipmakers are Arm customers and many of these companies, including Qualcomm Inc., Intel Corp. and Advanced Micro Devices Inc., sell chips that compete directly with products from Nvidia.

The FTC said in its complaint, filed in its administrative court, that the proposed merger would give Nvidia the ability and incentive to use its control of Arm’s technology to undermine its competitors, ultimately leading to reduced innovation, higher prices, and less choice.