Online music stores like iTunes could be dancing to a different drummer in France after the National Assembly approved a law Tuesday that would require their music to be playable on devices made by competitors.

The Assembly, France’s lower house, approved the bill by a vote of 296 to 193. The legislation also affects Sony and Microsoft, which also have used closed systems to sell music online. It is expected to go before the French Senate in May, when a final vote is scheduled.

The bill greatly affects consumers as well as sellers of online digital music. Apple’s iTunes makes up about 83 percent of legal audio downloads, according to the company. However, the only portable player able to run iTunes is Apple’s iPod, one reason for that device’s dominance over portable audio gadgets.

“If Apple breaks down, other markets could be affected,” said analyst Roger Kay, founder of the technology research firm Endpoint Technologies. “Black markets could arise to sell French [iTunes] subscriptions.”

If Apple licenses equal rights management software onto French iPods, that could result in a large number of “shipments of French iPods around the world,” Mr. Kay added.

An Apple spokesperson did not immediately reply to requests for comment.

U.S. investors appeared wary of the bill. Shares of the Cupertino, California-based company fell $2.14, more than 3 percent, to $61.85 in recent trading.

Under the legislation, companies would be required to reveal copy-protection technologies like Apple’s FairPlay format and Sony’s ATRAC3 code, used in Sony’s Connect online music store as well as its Walkmans and other players.

That could result in iTunes being playable for the first time on non-Apple portable devices, and non-iTunes songs able to run on iPods.

To be sure, diligent music lovers have been able to get around FairPlay. Tracks from iTunes are downloadable onto computers, so consumers are able to copy songs onto CDs, then make MP3 files from the discs.

But French advocates for consumer rights say users are unfairly inconvenienced when they can’t easily play music on whatever device they choose.

An American Thing

Mr. Kay, who lived in Europe for several years, noted the issue arises as much from culture clash as it does from business concerns. “They see it as an American company saying, ‘this is how we want to do business and that’s it’,” he said.

The bill also allows interoperability software. Content protected by proprietary technology can be rendered playable by such software (see iTunes Pour Tout Le Monde?).

But it also penalizes those who use the software for non-legitimate reasons like online piracy, and punishes those who disable copy-protection systems.

The law would be difficult for Apple to comply with, Mr. Kay said. “It’s not trivial for Apple to split iTunes and the iPod—it causes issues for both the online store and the hardware.”

Then again, iTunes was originally created to generate more sales of iPods.