WASHINGTON (AP) _ Millions of retirees are facing stiff increases next month in the premiums they pay for private health insurance to fill in Medicare's gaps.

The American Association of Retired Persons said the premiums on the Medigap policies for more than 3 million of its members are shooting up an average of 30 percent. The Prudential Insurance Company, which provides that coverage, is the biggest single seller of Medigap insurance.

Prudential executives blamed the increase on a soaring number of claims that its elderly customers are filing for medical services.

Most of the elderly buy supplemental coverage to cover things that Medicare does not pay for, including the $736 charged for the first day in the hospital and the beneficiaries' 20 percent share of doctor bills.

Peter Ashkenaz, a spokesman for the AARP, said the organization was concerned when it learned of the size of the premium increases. ``But we found we had to go with it,'' he said.

Lania Peterson, a Prudential manager in its Fort Washington, Pa., office, said the actual increases will vary from state to state. They also depend on which of the 10 different Medigap policies a person buys.

``Beginning in late 1994 we've seen a really significant increase in those medical costs and claims volume,'' Peterson said.

Prudential's Medigap claims jumped 38 percent in the first half of 1995, she said.

``We've tried to hold the line,'' she said. ``Our premium increases over the last five years average 7.7 percent annually. In 1992 and 1994 we gave premium refunds.''

AARP recently said it plans to terminate the agreement with Prudential when it expires at the end of December 1997. The organization said it will consider new options for selling supplemental insurance to members.

Medicare beneficiaries also pay a monthly premium of $46.10 to the government for their basic Part B coverage. That premium is scheduled to drop to $42.50 in January 1996. The Republican Medicare reform proposal before Congress would increase it to $53.50 a month.

Medigap policies typically cost anywhere from several hundred dollars to $2,000 or more a year. The most expensive policies offer some prescription drug coverage.

Diane Archer, executive director of the Medicare Beneficiaries Defense Fund, a New York-based consumer group, said consumers will be left ``between a rock and a hard place'' by the Medigap premium increases.

If they drop the supplemental coverage, their only real options are ``to self-insure, which is risky or impossible on a fixed income, or to join a Medicare HMO, which will limit choice of doctors and may impede access to necessary care,'' she said.

``Our concern is fee-for-service Medicare is going to become a luxury only for the most affluent seniors as Medicare supplemental premiums go up,'' she said.

Gail Shearer, a health policy analyst for Consumer Union, said the AARP Prudential policies are community rated, meaning that the insurer charges the same rates for all customers in a state regardless of their age or health. Some other Medigap policies are not sold that way.

She said that Prudential may be feeling the effects of ``adverse selection,'' in which younger, healthier senior citizens may be attracted to other policies that offer low rates for 65-year-olds, but raise them as people age.

``Prudential is competing on an unlevel playing field,'' said Shearer.

Medigap rates vary from state to state and from insurer to insurer.

Prudential's Peterson said the average monthly premium for the least comprehensive Medigap plan will go from $37 a month to $50 in 1996. The premium for a top-of-the-line plan will rise from $129 a month to $150, she said.

Other insurers are also filing for increases.

Paul Chvatal, 64, of Conklin, N.Y., who is on Medicare because of disability, said he was notified that his Blue Cross Medigap premiums were going up from $219 a quarter to $241 _ a 10 percent hike.

``It sounds like peanuts, but I don't have a pension,'' said Chvatal. ``I'm on a tight budget.'' He said he will probably opt for a cheaper plan with fewer benefits.