A private health advocacy group is calling for an immediate halt to the use of silicone implants for breast enlargement even as the Food and Drug Administration has started a review of the issue that will take at least two years.

In a letter to FDA Commissioner Frank Young released Wednesday, Dr. Sidney M. Wolfe, head of Public Citizen Health Research Group, also said the agency should issue an immediate alert telling the estimated 2 million women who have had such procedures they may run a heightened risk of health problems, including cancer.The agency has already scheduled a meeting of its plastic surgery devices advisory panel to review potential health problems that may be caused by silicone breast implants. It issued a background paper on the topic last week.

The FDA said it has invoked a procedure whereby silicone gel implants will have to undergo the type of scientific review normally associated with new medical devices even though the implants were already on the market in 1976. That was the year Congress gave the FDA authority over such products, which were given "grandfather" approval under the legislation.

The FDA said it decided "there are sufficient concerns about these products to call for premarket approval applications. This means that in order for the implants to remain on the market, manufacturers will have to supply FDA with scientific data demonstrating that they are safe and effective."