Projected growth in service and other private industry jobs could lead to a 66 percent increase in Washington area employment over the next 25 years, as the area population expands by nearly one-third, the Metropolitan Washington Council of Governments forecast yesterday.

More than half that growth will take place in inner-ring counties of Montgomery, Prince George's and Fairfax, COG said, while 40 percent of the population growth will take place in outer-county jurisdictions of Loudoun, Prince William, Charles and Frederick.

The new forecast projects 1.4 million new jobs in the area by the year 2010 -- an increase to 3.3 million from the 1985 total of 1.9 million. The report anticipates a 32 percent increase in population, from 3.4 million to nearly 4.5 million. In its last report, issued in 1985, COG forecast employment growth of 43 percent and population growth of 20 percent.

Although the overall job and population figures are slightly higher because Charles and Frederick counties joined COG after the 1985 projections for 2010 were made, increases over the earlier projections were based on COG's 1985 membership.

D.C. Council Chairman David A. Clarke said he believed a projected 29 percent increase in employment for the District was "optimistic," but said "anything could happen. We could have a major recession."

Martha V. Pennino, vice chairman of the Fairfax County Board of Supervisors, said the forecast of a 105 percent increase in employment and a 40 percent increase in population for Fairfax "means we really need to get busy and solve the problems we have," including inadequate roads and a shortage of housing.

The forecasts are part of a 12-year-old COG program aimed at providing local jurisdictions with realistic information about future growth that can be used to create regional plans for areas including transportation, housing and air quality.

In general, the metropolitan region has ample reason to be optimistic, said Robert E. Griffiths, acting director of COG's metropolitan development and information resources division, who presented the forecast during COG's annual meeting at the Loews L'Enfant Plaza Hotel.

Griffiths said an anticipated growth in private service jobs, such as computer services, research management and legal services, will "more than offset a decline in government employment."

He said such growth is supported by a new series of national economic projections that show a shift away from manufacturing and goods-producing sectors toward the service and information-producing sectors.

Some jurisdictions are expected to have the luxury of a substantial increase in jobs while experiencing smaller increases in population. For example, the District, Arlington and Alexandria are projected to account for almost 30 percent of the employment growth but only 2 percent of the population growth.

But Alexandria Mayor James P. Moran Jr. said that while he will be delighted to see growth, he also anticipates problems.

"It means that we are moving away from being a bedroom community," said Moran. "It is the best of worlds for affluent, well-educated, two-person families and the worst of worlds for large families with uneducated heads of households. They may have to move out of the area to find affordable housing."

Marvin J. Cetron, president and founder of Forecasting International, a Northern Virginia consulting company, and COG's keynote luncheon speaker, said the forecast failed to take into consideration a few factors that may have a major impact on the metropolitan area.

Cetron said that because of advances in computer technology, the country will experience a one-third reduction in secretaries and typists by 1995, a change that he said will be a blow to an area with a high concentration of office workers.

He also said that while about 88 percent of the country's jobs will be in the service sector by the year 2000, few high school students are being adequately trained to fill those positions.