A company that developed a blood test that detects dozens of types of cancer has acknowledged that about 400 of its customers were mistakenly told last month that they might have the disease.

The company, Grail, said in an emailed statement on Sunday that a vendor it works with had sent hundreds of letters with incorrect test results because of a “software configuration issue” that has since been resolved.

The letters went to customers who had recently purchased Grail’s Galleri test, which uses a blood draw to detect a cancer signal shared by 50 types of cancer and is available only by prescription.

The problem was not caused by inaccurate test results, Grail said. More than half of the people who received the letter in error had not yet had their blood drawn for the test, the company said.

The vendor, PWNHealth, notified Grail on May 19 that an “inaccurate form letter” had been sent to roughly 400 customers from May 10 to May 18, Grail said in its statement. The inaccurate messages were reported by The Financial Times.

After Grail was notified about the problem, it contacted the affected customers by phone and email, the company said. “No patient health information has been disclosed or breached due to this issue, and no patient harm or adverse events have been reported,” it said.

PWNHealth said in an emailed statement that, after it learned about the problem, it found that a system used to send template messages to people had a “misconfiguration.” The company did not specify how it learned about the issue.

“We addressed the underlying problem within an hour of becoming aware of it and have implemented additional processes to ensure it does not happen again,” the company said. “In partnership with Grail, we started contacting impacted individuals within 36 hours.”

The test result letters were erroneously sent out amid a regulatory battle between the United States and Grail’s parent company, Illumina, the leading maker of gene-sequencing machines. Illumina acquired Grail in August 2021.

In April, the Federal Trade Commission ordered Illumina to divest itself of Grail because the acquisition could “stifle competition and innovation” in cancer testing, raising prices and shrinking choices for consumers.

Illumina said it would appeal the F.T.C. ruling and a similar regulatory challenge by the European Union. The company said in April that winning both appeals would allow it to make the Galleri test more widely available, as well as more affordable and profitable.

If its appeals fail, Illumina will “move expeditiously to divest” itself of Grail, the company said.