A key advisory panel recommended yesterday that the government stop Xraying most of the 270,000 women in a program to detect breast cancer, because of concern that the risks of Xrays may outweigh benefits.

The recommendation - made to the National Cancer Institute by its Cancer Control Advisory Committee - is virtually certain to be adopted by the NCI in guidlines to directors of the 27 medical centers where the Breast Cancer Demonstration Project is conducted.

The recommendation could herald demise of the program. Whether it will, or whether the program will be continued and if so, in what form, will be discussed in September after another group of NCI advisers makes a recommendation on its mammography policy.

Yesterday's development mainly concerned participants in the 35 through 49 age group who comprise about 75 per cent of the 270,000 enrolled women.

Critics of the increasingly controversial program have contended that in under 50 women, the X-rays called mammograms, may be causing more new cases of breast cancer than preventing deaths.

The advisory committee, in a unanimous action, recommended that mammograms be given hereafter only to the small fraction of under 50 women who have high risk of breast cancer. Such women either have had single mastectomies, or have mothers or sisters who have had them.

The recommendation follows advice given the NCI in March by three task forces of outside consultants appointed in October 1975.

The same task forces, troubled by the inconclusiveness of data on the benefits of routine mammography for older women, also recommended that it be phased out for participants 50 and older. In its place, there should be controlled clinical trials to determine precisely what is the benefit risk ratio in older women, the consultants said.

Yesterday, however, the Cancer Control Advisory Committee, as part of its unanimous action, recommended continuation of routine mammography in over 50 women.

The committee also said a woman coming to a medical center with a private physician's recommendation for a mammograms should be cautioned and should be required to sign an informed consent form. The decision whether to proceed should be the center director's not the physician's the panel said.

The NCI has paid more than 30 per cent of the $54.6 million cost of the screening program, which was conceived by the American Cancer Society.

The ACS, which has a close relationship with the NCI, hoped the program would increase early detection and treatment of breast cancer, which is the leading cause of death among women 40 to 44. The discase causes about 33,000 deaths a year.

In Sunday's Post Outlook section science writers Daniel S. Greenberg and Judith E. Randal said there is little dispute about justification of mammography for women at high risk. But, they wrote, promotional campaigns for the program encompass about 30 per cent of all women under 50.

"No attempt has been made to explain the mathematical absuridty of why - when women have an 8 per cent risk of breast cancer in an entire life time - 4 out of 5 should be at high risk before reaching age 50," they said.

In New York City, the ACS said it expects its advisory committee will endorse the NCI panel's advice.