Doctors at Bethesda Naval Hospital found a "large" precancerous growth in President Reagan's lower intestine yesterday and scheduled surgery for midday today to remove it, White House spokesman Larry Speakes said.

This type of growth can develop into cancer in about 10 percent to 20 percent of cases, a cancer specialist said yesterday. The risk grows as the polyp becomes larger. Speakes said doctors called Reagan's polyp "large."

"The president is in superior condition, and chances for a normal and full recovery are excellent," Speakes quoted Dr. Dale Oller as saying. Oller, head of general surgery at the naval hospital, is scheduled to perform the operation today.

The surgery, which involves an incision through the abdomen, normally takes about three hours and must be performed under a general anesthesia. Speakes said Reagan would remain in the hospital for seven to 10 days.

He said doctors described the growth as "a large villus adenomatous-appearing polyp," but he could not say exactly how large. The polyp was found in the cecum, the part of the colon farthest from the rectum, and near the juncture between the colon and the small intestine.

A portion of the growth was removed for laboratory analysis to determine its true nature. If the test results, which are expected some time today, confirm that the polyp is a villus adenoma, it would be of the type of colon abnormality that is most likely to progress to a cancer.

Regardless of the laboratory findings, Reagan's doctors recommended that the polyp be removed surgically in an operation called a "formal right colectomy." The operation involves removing the segment of intestine from which the polyp is growing and rejoining the severed ends.

Doctors gave Reagan the option of returning to the hospital in two or three weeks for the operation or of having it done immediately. Reagan, Speakes said, opted to stay in the hospital and have the operation today.

Speakes said that Reagan was resting comfortably at the hospital, undergoing normal preoperative preparations, and that he was in good spirits. When he entered the hospital earlier, Reagan, wearing a western-style shirt and casual slacks, waved to reporters and said he felt "fine."

The polyp was found during a special examination that doctors had recommended last March when a routine medical exam revealed the presence of a small inflammatory polyp that proved to be benign and much like a polyp discovered a year earlier. Both were in the lower part of the intestine, which can be examined relatively easily with a procto-sigmoidoscopy. This is a procedure of inserting a tube a short way up the intestine from the rectum.

After finding the second polyp, doctors recommended that it be removed during a more elaborate procedure that could also examine the entire six feet of the colon for any additional growths and remove them as necessary.

Under heavy sedation but not an anesthetic, this procedure was carried out yesterday afternoon, from "This type of polyp has a somewhat low probability of becoming cancerous, but if you leave it alone, it is very likely to progress into cancer ." -- Dr. Lance Liotta 2:12 to 3:20 pm. The known polyp was removed with the examining instrument, which has a flexible tube containing a fiber-optic viewing apparatus and a device at the end for snipping off polyps. The instrument was then maneuvered through the entire length of the president's colon. The only additional abnormality found was the large polyp at the innermost end of the colon.

Because the polyp could not be removed with the device, doctors only removed a small portion of it for analysis and recommended the surgery that is set for today.

"This type of polyp has a somewhat low probability of becoming cancerous, but if you leave it alone, it is very likely to progress into cancer ," said Dr. Lance Liotta, chief of pathology at the National Cancer Institute.

Liotta, who specializes in the mechanisms by which abnormal growths become cancerous and spread, said there was a very slight chance that laboratory analysis of the removed polyp could show that a few cells had actually turned cancerous. If these were still deep inside the polyp, the chances would be very good that they had not spread. But if cancerous cells were found on the outside, the prognosis would be poorer.

"You can never really tell about this until you go in surgically and have a good look," Liotta said. "There's a chance they could find more during the operation or, more likely, after a microscopic examination of a section of the polyp."

In the unlikely event that cancer cells are found, but that the surgery removes all the colon wall to which such cells may have spread, the chances of surviving more than five years are greater than 90 percent, according to Dr. Marshall S. Bedine, a specialist in intestinal diseases at Johns Hopkins School of Medicine.

But, Bedine said, if the polyp growth has gone into the colon wall, the surival rate declines to about 40 percent or 50 percent in the five years after surgery.

The colon is the lowermost segment of the intestinal tract except for the rectum. Its chief function is to remove water from bodily wastes before they are excreted. Food is digested in the stomach and passed into the upper, or small, intestine, where nutrients are absorbed into the bloodstream. The first part of the lower intestine, or large bowel, is called the cecum.

Several cancer specialists not connected to the Reagan case have privately expressed surprise that the type of procedure done yesterday was not performed as soon as the first polyp was discovered in May of last year. They also said any large growths would probably have shown up in an X-ray if the president had been given a barium enema. This is a fairly routine procedure that makes tissue shapes in the colon show up on an X-ray.

These doctors, who asked not to be identified, stressed, however, that Reagan's doctors may have information of which they are not aware.