The Food and Drug Administration has objected to a letter sent last week by a manufacturer of silicone-gel breast implants advising doctors that approval of its controversial product was imminent, causing Mentor Corp. to send out a second letter two days later retracting the first.

The letters, and the agency's response, indicate the long regulatory battle over silicone implants may be nearing an end and that the companies that make the implants are jockeying for position as a lucrative market reopens after 13 years of tight sales limitations.

The faxes sent to plastic surgeons included a cover letter telling them to make sure their patients knew that the silicone gel implants had been approved and urging them to stay enrolled in a long-running clinical trial of the implants. A second page, described by Mentor as "approved by the FDA," details some conditions attached to the approval.

Among them was a restriction against selling silicone implants for breast enlargement to women younger than 22. The implants would be available to women of any age for breast reconstruction surgery because of cancer or injury.

The FDA did not formally comment on the Mentor letters, except to say that no formal decision has yet been made. But an FDA official, speaking on the condition he not be identified because of the sensitivity of the issue, confirmed that agency officials were concerned about the Mentor letters, and said that the information in them is incorrect.

Josh Levine, Mentor's president and chief executive, said yesterday that the initial letters had been drafted to be sent "upon notification of a potential FDA decision" and that Mentor's application "is still under review and has not yet been approved."

"This correspondence was mistakenly sent to the study investigators," he said of the letters. "Upon learning of the error, Mentor took steps immediately to correct the mistake. . . . We have resolved the issue to FDA's satisfaction."

Mentor and Inamed Corp., both of California, received "approvable" letters from the FDA this year indicating that their products would be allowed on the market if certain conditions were met. The agency did not give any time frame for a final decision.

Sales of silicone-gel breast implants were restricted by the FDA in 1992 because of concern that they might cause immune system disorders. Subsequent study has lessened concerns over long-term dangers, but a contentious battle over the implants' use continued.

The FDA has held several lengthy advisory panel hearings in recent years, which featured emotional testimony by opponents who alleged terrible harm from silicone implants and pleas from supporters who said all women -- and especially women undergoing reconstruction after surgery -- should be allowed to choose silicone implants. Silicone implants are generally preferred over the saline implants on the market.

Some of the major reservations voiced by the FDA and advisory panel experts involved the relatively limited amount of long-term information about the implants' effects on women's bodies. Both Mentor and Inamed have committed to continuing studies of future complications if their implants are approved. The unusual letters sent out last week by Mentor involved efforts to make sure patients remain in their studies.

The cover letter began: "In anticipation of a final gel breast approval, the FDA is requiring that [study] doctors send a letter to their study patients notifying them of the gel approval, and to remind them of the importance of their commitment to continue their 1-, 3-, and 5-year follow-up visits." The letter was signed by Rosalyn Cole, manager for the studies.

Her letter described an attached letter, titled "SAMPLE LETTER -- Contingent on Final FDA Approval of Devices," saying it had been approved by the FDA.

The Mentor letters were criticized yesterday by Nick Teti, president of Inamed.

"We are very disappointed to learn about this situation, given how this clearly has a disruptive effect on the patients and physicians," he said.