Even if Metro raises subway and bus fares every two years, Washington area local governments still will need to spend more than three times as much as today to keep the transit system operating within 20 years, a Metro committee was told yesterday.

Faced with such a staggering increase in costs, several Metro board members said local governments probably will be forced to establish a special tax -- such as a levy on sales or income -- to finance Metro operations, even if fares continue rising.

"One of the greatest challenges we need to look at in the next two to three years is finding some method of having a dedicated source of revenue in each of the jurisdictions," said board member Joseph Alexander of Fairfax County. "Otherwise, the local jurisdictions won't be able to support the system."

The call for a special tax for Metro comes as the local economy is in a slowdown and the region faces other pressing transportation needs that also would cost billions of dollars.

So far, there is no regional consensus on how to pay for the projects.

According to a draft of Metro's long-term plan released yesterday, the contribution from local governments will increase from about $300 million a year now to more than $1 billion in 2010, even with a 5 percent fare increase every two years.

The main reason for the increase is that the planned 103-mile Metrorail system is expected to be complete by 2003.

The completed system would have substantially higher operating costs because the number of rail passengers would jump from over 500,000 today to 1 million by 2010.

The federal government's share of financing also is expected to drop at a time when the system is aging and maintenance expenses are rising.

Like most transit agencies, Metro cannot cover its operating expenses with the money it gets from fares, so federal, state and local governments contribute to make up the difference.

The subsidies have increased in recent years, leading to pressure on Metro from the participating governments to cut spending and raise fares.

The District has missed two of its payments this year, forcing Metro to borrow money to meet its expenses.

Gladys W. Mack, a Metro board member representing the District, said yesterday that discussion of a special tax "won't become a reality until it becomes a necessity. And the necessity is on its way."

When Mack was board chairman five years ago, she said, the Metro board promoted the idea of a special tax for Metro but was politely rebuffed by area elected officials skittish about asking residents to cough up more money.

Other calls for a special tax also have fizzled.

Before leaving as Metro general manager in 1983, Richard S. Page recommended that local governments support taxes for Metro.

The plan released yesterday is a guidebook for the board to use in developing a final strategic plan. It raised other serious questions about Metro's future finances.

Total system costs are expected to nearly double, to $2 billion, in 20 years, the plan said.

The annual cost of running the rail operation is projected to go from the current $270 million to $1 billion in 20 years, the plan said, while revenue would rise to $725 million, even with a fare increase every two years.

Bus costs represent an even bigger problem, going from just under $300 million this year to $850 million in 2010, while revenue is expected to stay relatively constant at about $100 million.

Revenue from fares also may be less than expected.

The rail estimates are based on an increase of 4 percent a year in ridership; the increase has been averaging only about 2 percent a year recently.