The key factor in the Washington area's economy, once seen as riding on the broad shoulders of the federal government, is, and will continue to be, the dominance of service-sector jobs, according to two reports from the Washington Metropolitan Council of Governments.

The number of jobs in the service industries increased by 146,000, or 33 percent, between 1980 and 1985, according to a recent COG report. During the same period, the number of jobs in the federal government declined by 3 percent.

"Through World War II, a majority of all workers here were employed by the federal government," said Jay Langford, head of COG's department of planning and analysis. "But in 1985, the government accounted for fewer than 1-in-5 workers."

For every federal job lost in the area during the last five years, there were 13 new service-sector jobs, the report said.

"The services sector is the largest segment of the Washington area's economy . . . and that makes us similar to places like New York, Dallas and San Francisco," Langford said.

Another preliminary COG analysis projects that jobs in the service industries will continue their amazing growth in the area, increasing by 39 percent during the next 15 years.

Service industries include those in the medical, legal and business fields.

The number of retail jobs also may soon exceed the number of government jobs, according to the report. There were 310,000 people employed in retailing in 1985, compared with 359,000 employed by the federal government, according to the report.

The phenomenal growth in service industries that has drawn new residents to the Washington area also has resulted in the area's most rapid growth since the late 1960s; most of that growth has been in the Maryland and Virginia suburbs.

That population distribution is expected to continue during the next 15 years, according to the COG forecast, which predicts that two of every three new households will locate in the outer suburban area.

As more households are added in the region beyond the Capital Beltway, local officials echo the conclusion of the COG reports that local services will be stretched to meet the needs of the newcomers -- particularly in the area of transportation.

In the next 10 years, getting from the new outer suburbs (such as Spotsylvania County) to the inner suburbs (such as Fairfax County) will be the newest transportation problem, said Fairfax County Board Chairman Jack F. Herrity.

The Washington suburbs are expected to continue to grow until the year 2000, and some household growth also is expected in the 10-mile core area that includes the District, Arlington and part of Alexandria, according to the report. There has been almost no growth in that area in the past 15 years.

Other conclusions of the reports:

*Unemployment will continue to be highest in the central areas of the region, which will make it difficult to match the unemployed with new jobs in the outer suburbs.

*Additional capacity for sewage-treatment facilities will be needed.

*By the year 2000, there will be approximately 75,000 more children in schools than there were in 1980.

*The percentage of mothers with children under the age of 6 in the labor force will increase by 23 percent by 1990, which will put greater pressure on already severely taxed day-care services.

*The over-65 age group will continue to be one of the fastest-growing segments of the population.