Last week the FDA banned in the U.S. market the use of drug ingredients from Ranbaxy's Toansa plant. AFP/Getty Images

NEW DELHI—Workers at a Ranbaxy Ltd. drug plant repeatedly fudged test results to make it appear that raw materials and active pharmaceutical ingredients met required standards when they didn't, according to a report by inspectors from the Food and Drug Administration.

FDA officials visited Ranbaxy's Toansa factory in the northwestern Indian state of Punjab early this month and said they discovered workers retesting "until acceptable results are obtained" and deleting evidence of failed tests.

Last week the FDA banned in the U.S. market the use of drug ingredients from Ranbaxy's Toansa plant—a serious blow for Ranbaxy, since Toansa supplies many of the critical ingredients used in the company's generic drugs. "This development is clearly unacceptable and an appropriate management action will be taken upon completion of the international investigation," Ranbaxy Chief Executive Arun Sawhney said last week in response to the FDA ban.

Ranbaxy didn't respond to requests for further comment on Monday.

The FDA order could result in supply disruptions of several Ranbaxy generic drugs in the U.S., including fenofibrate tablets, which are used to reduce certain fatty substances in the blood and to increase good cholesterol, as well as valacyclovir, which treats shingles and genital herpes.

Ranbaxy holds 6% of the U.S. fenofibrate market and 17% of the valacyclovir market, according to Barclays. Competitors, including Mylan Inc. and Teva Pharmaceutical Industries Ltd. , also sell generic versions of these products.

Ranbaxy holds a 71% share of the U.S. market for hydroxychloroquine, which is used to prevent and treat malaria, but there is a relatively small market for that drug. Other suppliers include Par Pharmaceutical Cos. and Lupin Pharmaceuticals Inc.

On Monday, an official of Japanese drug maker Daiichi Sankyo Co. , which owns more than 60% of Ranbaxy, said the Japanese company had sent employees to India to "help support Ranbaxy toward the solution of the ongoing problem." He declined to provide further details.

The FDA inspectors also noted that analytical and microbiological laboratories at the plant were in "significant disrepair," with windows that couldn't close and a sample-preparation room with flies that were "too numerous to count."

In one lab, the FDA report said, inspectors "identified the presence of numerous sticky notes" that were found to "contain instructions for corrections to be made to the raw data" for testing. The inspectors also wrote that they had observed a worker in the quality-control lab backdating a log entry.

"We immediately questioned this analyst regarding the reason for backdating his record, who responded that he had only entered '2014,' despite our visual observation of him entering a signature and full date entry a few moments earlier," the report said.

In January 2012, Ranbaxy reached a consent decree with the Justice Department over manufacturing quality issues. The decree required the company to take steps to ensure the integrity of production at three plants in India—not including the one in Toansa—and one in New York.

In September the FDA blocked imports from Ranbaxy's newest plant, in Mohali in northern India.

Until the problems at Toansa can be fixed, industry analysts said, Ranbaxy would likely need to outsource the production of active pharmaceutical ingredients to other companies approved by the FDA so that it can keep exporting medicines to the U.S., the source of 40% of the company's revenues.

"That means the cost competitiveness of the company goes down," said Hemant Bakhru, an analyst at CLSA in Mumbai. "They'll need time to start sourcing new products and notify the FDA. That'll take several quarters, so there will be some disruption of supplies in between."

That could mean further pressure on Ranbaxy's already thin profit margins, Mr. Bakhru said.

"It's very clear that top management has been working over time to sort this issue out," said Nitin Agarwal, an analyst at Mumbai's HDFC-SSKI Securities.

"Somewhere, the chain of command is not working. Guys at shop level are continuing the same practice that got them into trouble in the first place."

The FDA said it is evaluating whether drug shortages will result from its action. If the agency determines that a medically necessary drug is in shortage or at risk of shortage, it may modify the order against Ranbaxy to preserve patient access to drugs manufactured "under controls that are sufficient to assure quality, safety and effectiveness."

—Peter Loftus and Hiroyuki Kachi contributed to this article.

Write to Sean McLain at