Sen. Charles H. Percy (R-Ill) linked to the controversial liquid protein diets, called yesterday for an immediate ban on over-the-counter sales of diet product.

Percy said the new figures, bringing to 26 the deaths that may be related to liquid protein diets, make urgent the need for a Food and Drug Administration ban on the product.

In a harsh letter to FDA Commissioner Donald Kennedy, the senator said the agency should order all predigested liquid protein substances off the market until they are proven safe and effective.

Only after thorough testing should the popular substances, known widely and used possibly by millions as part of a "last-chance" diet, be returned to the market - and then as prescription drugs, Percy said.

Liquid protein, marketed under a wide variety of brand names, is derived from animal tendons, cartilage and hides. It is used in combination with vitamin and mineral supplements - often with no other food - to obtain quick, large loss of weight.

Earlier this month, Kennedy said that at last 16 deaths and a number of severe illnesses "may be associated with the use of the predigested liquid protein diet."

The federal Center for Disease Control (CDC) in Atlanta, which yesterday confirmed Percy's new figures, said that 10 of the 16 deaths reported earlier involved women with no history of heart disease who died suddenly of heart disease who died suddenly of heart irregularities. All 10 were dieting under physicians' care.

In a Nov. 9 conference, Kennedy said his agency was carefully reviewing the situation and, among other things, preparing a mandatory label to warn that the protein be used only under medical supervision.

John T. Walden, assistant FDA commissioner, said yesterday that the agency is moving ahead with its review and will propose mandatory label wording "within a matter of days."

Walden rejected Percy's charge that FDA is acting indecisively and insisted that the senator's leter contained "essentially no information . . . that we are not aware of and acting on."

But Percy said that the FDA approach - warning labels and urging dieters to be cautious - is "an inadequate reponse to what appears to be a highly serious health danger."

Percy said he has asked the staff of the Senate Permanent Investigations Subcommittee, of which he is the ranking Republican, to consider a public inquiry if FDA fails to move more forcefully.

Walden said the FDA is working with the CDC to get more data, consulting with medical experts, inspecting plants to test liquid protein quality control, checking labels for accuracy and preparing an advisory bulletin with more information for health professionals.

"We are on top of this," Walden said. "There clearly is nothing wrong with the protein supplements. It is the way they are promoted and used."

The liquid proteins, classified as food by FDA, are exempt from most regulation. FDA may, however, inspect production, regulate labeling and seize tainted products.

Percy said no reliable estimates are available on the number of dieters using liquid protein products, but he suggested there may be hundreds of thousands of them.

He noted that "The Last Chance Diet," the book by Dr. Robert Linn that popularized liquid proteins and the modified-fast approach, has sold more than 2 million copies.

Linn, a Philadephia physician, also operates a weight-loss clinic in Washington, where the liquid-protein diets are administered under medical supervision.

Percy said, 10 women whose deaths were studied by the CDC "started the so-called 'last chance diet' out of hopes of losing weight and instead lost their lives."

"How many more persons must die or become seriously ill before the government moves forcefully in this matter?" he asked Kennedy.