First Lady Nancy Reagan entered Bethesda Naval Hospital last night for a biopsy of a "suspicious lesion" in her left breast and the White House announced that doctors plan to remove the breast today if the lesion is found to be cancerous.

Informed sources close to the Reagans said the discovery of the lesion Oct. 5 has caused President Reagan deep concern and preoccupied him at a time of intensifying difficulties in his administration, including conflict in the Persian Gulf and the doomed campaign to put Judge Robert H. Bork on the Supreme Court.

Intimates of the president have said that even more than his own bouts with cancer and the 1981 assassination attempt, the one health problem certain to distract him would be a serious illness or surgery involving his wife. These sources, who have had regular contact with Reagan, said concern over the operation had diverted much of Reagan's energy and attention from his official duties in recent days.

White House spokesman Marlin Fitzwater said the lesion was found by a routine mammogram during Nancy Reagan's annual physical. Her spokesman, Elaine Crispen, said Nancy Reagan, 66, was told of the discovery Oct. 6 shortly before a White House dinner for the crown prince and princess of Japan.

Fitzwater said "there is the hope" the lesion is benign, but he said there are "any number of medical indicators" suggesting that the First Lady will have to undergo the operation to remove the breast, known as a modified radical mastectomy. "There is an assumption that there probably will be cause to proceed with the operation," Fitzwater said.

Using local anesthesia, doctors will insert a needle as a marker into Nancy Reagan's left breast and a tissue sample from the "suspicious area" will be removed for a biopsy, according to a White House statement. If there is "microscopic evidence" of cancer, the doctors will go ahead with the removal of the breast.

The president accompanied his wife to the hospital last night and, after returning to the White House overnight, planned to return for the tests and possible surgery today. Reagan held his wife's hand as they left the White House. The president then turned to escort Dr. Richard Davis, Nancy Reagan's brother, a Philadelphia neurosurgeon, as they walked to a helicopter. Mrs. Reagan, looking pensive, gestured to reporters but said nothing as she then grasped Davis' hand. Davis was to return to the White House last night with the president and accompany him to the hospital today.

Despite what the sources described as their private worries, the Reagans sought yesterday to show they are taking the news in stride. Nancy Reagan, who took a prominent role in looking after the president in his health crises, said when the lesion was discovered, "I guess it's my turn," according to Fitzwater.

"Well, of course I'm concerned, and so is she," the president told reporters yesterday. "But, at the same time, I have great confidence in the medical personnel who are in charge of this case."

Nancy Reagan went ahead yesterday with a planned trip to the first New Hampshire Foster Grandparents conference, where she made no mention of the possible surgery. Fitzwater said she postponed the hospital visit for 10 days because she wanted to keep a number of commitments, including a White House dinner for El Salvador President Jose Napoleon Duarte, and the New Hampshire event yesterday.

"She is in good spirits. I think she has some anxiety, as anyone would, about an operation of this kind, but generally feels very positive about it," Fitzwater said.

Crispen described Nancy Reagan as "anxious for tomorrow afternoon to come so that it is all behind her." She added, "In her mind she is resigned to the fact that if they have to take the second step, she is prepared for that. She is not going to be shocked."

Crispen said of the president, "I worry about him. She's his little girl. He's seen her do it {keep a hospital vigil} enough times. Now he can play nurse. It's her turn and it's also his turn."

In the written statement, the White House said "all other alternatives" for dealing with the lesion had been discussed with Nancy Reagan and "she accepts the procedure recommended for her circumstance." The statement added, "Further recommendation for postoperative treatment will be made pending the type and stage of the disease encountered."

The physicians participating in the procedures are Donald McIlrath, professor of surgery at the Mayo Clinic, and Oliver Beahrs, president of the American College of Surgeons and a specialist in breast cancer, along with John E. Hutton Jr., physician to the president, and Capt. Harry B. Etienne, chief of general surgery at the naval hospital.

Breast cancer is second only to lung cancer as the leading cause of cancer deaths among women. The American Cancer Society estimates that 41,000 women will die of the disease this year and that 130,000 new cases will be diagnosed.

Women at greatest risk of developing the disease are over 50 years old or with a family history of breast cancer. For older women, lesions such as the one that doctors discovered in Nancy Reagan's breast are more likely to be cancerous than they are in younger women, according to medical experts.

But discovering a lesion through a mammogram, and before it can be felt, usually means that the disease has not spread to the rest of the body.

The surgical risks for women having modified radical mastectomies are few. The surgery is not complex. There is little loss of blood. And most patients, regardless of their age, are out of the hospital within a few days.

Crispen said Nancy Reagan did not want her children and their families told until this week because she did not want them to worry.

The president taped his weekly Saturday radio address, to be broadcast today. White House officials originally gave reporters a schedule for the president yesterday showing that he was going to Camp David in the afternoon. Fitzwater said he knew Thursday that his wife was going to the hospital instead, and put out the wrong schedule "because we did not want to announce this operation until all the preparations were made and all the scheduling had taken place that needed to occur in terms of the hospital."

Staff writers Donnie Radcliffe, Michael Specter and Susan Okie contributed to this report.