Elderly patients who belong to private health plans will have to pay three times as much in monthly premiums next year as they do now and will find HMOs significantly less willing to pay for their medicine, according to a government analysis released yesterday.

And a growing number of older Americans, particularly in rural areas, are unable to join a managed-care plan because none exist where they live, according to the analysis of the HMOs that provide care to 6.3 million people on Medicare.

The trends cited in the new report, combined with the withdrawal of dozens of HMOs from Medicare since last year, are fueling a perception in the Clinton administration and among some in Congress that the managed-care industry is an unstable partner for the government. That perception is deepening even as others on Capitol Hill believe that greater competition among private HMOs is crucial to ensuring that Medicare does not run out of money early in the next century.

"We can't count on managed-care plans to provide stable and secure coverage to the Medicare population," said Diane Archer, president of the New York-based Medicare Rights Center. She said the center is deluged with phone calls from elderly people who have been dropped by their health plan or are no longer able to afford their medical care.

According to the new report by the agency that runs Medicare, the average premium that elderly people must pay each month for health plans' basic services will triple from $5.35 this year to $15.84 as of January. The number of plans that charge no monthly fee is diminishing.

The most dramatic reduction in benefits for Medicare patients involves prescription drugs. In a sharp reversal of recent trends, none of the 262 HMOs that accept Medicare patients next year will cover the full cost of those patients' medicine. In addition to requiring patients to pay for part of each prescription, many plans are setting new limits on how much medicine they will cover, with nearly one-third refusing to pay for more than $500 worth of drugs a year.

Those findings are based on information HMOs had to give the Medicare agency in July to let federal officials know whether they were going to take part in Medicare in the coming year and--if they were--what services they would offer and how much they would charge.

The report officially was released yesterday by Vice President Gore in an appearance before the American Medical Association. Although billed as an appearance in his capacity as vice president, Gore also reminded the audience of hundreds of physicians from around the country of a separate health care proposal he issued two weeks ago as part of his presidential campaign.

Gore and other administration officials said the analysis underscores the need for a Medicare reform plan President Clinton recommended this year, which calls for prescription drug coverage for all people age 65 and older, regardless of whether they get care through HMOs or through the traditional Medicare program.

But Susan Pisano, a spokesman for the American Association of Health Plans, a large trade group for HMOs, said the trends underscore a need for Congress to give health plans more money so they can offer more generous benefits. The government's analysis, however, disputes the industry's argument, saying there is no evidence that HMOs are most prone to drop out of the program in parts of the country where Medicare pays them the least.