LONDON — Syngenta, a Swiss chemicals company, produces one of America’s most popular herbicides. It is called atrazine, and 73.7 million pounds of the chemical compound were applied in the United States in 2013. It was used on more than half of all corn crops, two-thirds of sorghum and up to 90 percent of sugar cane.

But Syngenta cannot sell atrazine to farms in its own backyard.

The weed killer is banned as a pesticide in the European Union as well as in Switzerland over concerns that it is a groundwater contaminant.

Syngenta, however, did not get the memo.

Even though the European Union banned atrazine over a decade ago, the company has long insisted that the pesticide was not banned. On one corporate website, Syngenta points to “anti-atrazine activists” who “claim that ‘atrazine’ is banned in the European Union. This is patently false.”

Another Syngenta-backed site, “Saving the Oasis,” also blames “anti-atrazine activists.” And another such site, AGSense, says, “We’ve known it all along, and now you know it too: Atrazine is not banned in the European Union.”

The company has repeated its assertion to reporters.

“It is not banned,” Ann Bryan, a spokeswoman for the company, said in an email, though she acknowledged that “countries in the E.U. currently do not use atrazine.”

Companies are perhaps understandably sensitive about revealing too much about the gulf that exists between American and European regulation of pesticides and other chemicals.

Generally speaking, the European approach incorporates the so-called precautionary principle and requires companies to establish that new chemicals are safe before they are put on the market. The American approach puts the onus on regulators to show some evidence of danger before taking action against new chemicals.