A task force of the Washington Center for Metropolitan Studies recommended yesterday that Metro design a basic regional transit system that would meet almost all its costs by regularly increasing fares - something no transit system in the United States is achieving today.

Any single local government could choose to subsidize fares for poor or elderly riders and could choose to add subsidized additional bus service to the regional network, but would do so at its own costs apart from the Metro budget under the task force proposal.

If Metro and the localities that make up the transit partnership here continue their present fare and service politicies, and if projected ridership slips by as much as 20 percent below 1990 estimates, the annual Metro operating deficit then will be $500 million, the report said. It is expected to be $100 million in the fiscal year beginning July 1.

The recommendations of the task force, a group with a strong academic and financial orientation, are in no way binding on local officials, but are nonetheless certain to be controversial.

For one thing, they focus attention on a critical question in the long range Metro financial picture: how to pay for operating the transit system once it is built.

The federal government has insisted that local governments complete by Aug. 31 a financial plan that includes proposals for both paying operating and construction costs of Metro before futher construction money will be approved. A policy on fare levels has been a key sticking point in regional negotiations.

The most controversial task force recommendations are those concerning fares and subsidies for the poor and other "favored riders," as the task force choose to call them.

That is because an underlying assumption of the self-supporting Metro system is that fares will escalate along with inflation. Experience at Metro has been that fares escalate at about half the rate of inflation and any fare increase is bitterly fought, particulary by the District of Columbia government.

Special subsidy programs for the poor, as opposed to universal low fares, would deal with the D.C. government's problem, in the task force's view. But D.C. Council member Jerry A. Moore. Jr., who attended a day-long briefing on the report, said special subsidies to individuals "stigmatized" them and was unacceptable in his view. Other city officials have expressed similar views.

The task force recommendation for a "basic transit system" carries with it the implication of what transit experts call "peak-hour skimming," or bus and subway operations designed to carry only high-denstiy loads and to ignore other transit needs.

Localities would provide their own subsidized service - perhaps on contract with Metro - if they wanted more than the basic transit needs.

"Those social service routes have to be financed, too," Metro general manager Theodore C. Lutx commented during the task force presentation.

While those questions will not go away, some of the basic findings of the task force during its eight-month effort will add to the quality of the debate.

Key among them is an emphasis on staudies that have shown that the number of rush-hour transit riders is more likely to increase in response to good service than it is in response to lower fares. Conversely, higher fares produce more revenue in a tradeoff with a relatively small drop in ridership.

Richard H. Pratt, a transportation consultant, told the group of about 85 area officials that a "quick and dirty" estimate for Washington based on experience elsewhere would indicate that a 1 percent increase in fares should result in only a .2 percent decrease in riders.

Vivian E. Watts, a member of the task force and director of legislative activities for the Fairfax County Chamber of Commerce, asked how that would work on a year-to-year basis.

"Does that mean an annual increase of 6 percent in fares would mean a 12 percent ridership loss in a decade?" she asked. There was no definitive answer.

The task force was led by Bertil W. Johnson, the respected, retired Arlington County manager. The eight-month study effort was financed by grants from ARCO, the Ford Foundation and the Eugene and Agnes E. Meyer Foundation.