WASHINGTON — The federal government on Friday set a standard for gluten-free claims on food labels, a step that health officials said would help the three million Americans with celiac disease and bring uniformity to the $4 billion market for gluten-free products.

Gluten is a composite of starch and proteins found in certain grassy grains like wheat, barley and rye. When eaten by people with celiac disease, gluten can trigger the production of antibodies that damage the lining of the small intestine.

To protect people with the disease, Congress passed a law in 2004 calling on the Food and Drug Administration to set standards for how much trace gluten could be in foods whose labels said they were gluten free. A standard became even more urgent, observers said, when broader consumer demand for gluten-free foods drove the rapid expansion of the market.

The F.D.A.’s publication on Friday of the final rule is the culmination of that process.

The agency set a gluten limit of 20 parts per million in products labeled gluten free, said Michael R. Taylor, deputy commissioner for foods and veterinary medicine at the F.D.A. The limit had long been discussed and did not come as a surprise to industry or patient advocate groups. It was similar to the level adopted in recent years by the European Union and Canada, Mr. Taylor said.