Consumer prices rose 1.2 percent in September, equivalent to an annual rate of 14.8 percent, the Labor Department reported yesterday.

September marked the third consecutive month of large price increases, despite the economy's drift into recession. The consumer price index for September was 279.3, which means goods and services that cost $100 in 1967 cost $279.30 last month.

Separately, the government reported that prices in the Washington area rose 1.8 percent during August and September, a rate of inflation equal to the rise for the previous four months. Details on Page D9.

September's price increases occurred throughout the national economy, although food and housing costs continued to lead the rise. On Wall Street, stock prices fell sharply yesterday, a decline analysts attributed partly to the new consumer price figures. The Dow Jones average of 30 industrial stocks declined 10.28 to 837.99.

Housing costs across the nation jumped by 1.3 percent in September, as mortgage interest rates last month climbed to an average of just under 18 percent. Sharp rises in most grocery store food prices were offset partially by cheaper poultry and fresh vegetables, leaving an overall rise in food and beverage prices of 1 percent.

Transportation costs rose 1.2 percent while other goods and services, including higher charges for school tuition and supplies, soared by 1.8 percent.

Murray L. Weidenbaum, chairman of the president's Council of Economic Advisers, said last month's consumer price index "emphasizes that inflation continues to be a serious problem."

But Weidenbaum said the September figures are somewhat distorted by the treatment of housing, which tends to overstate changes in mortgage interest rates. Weidenbaum said that "over the last several months, the CPI has been heavily influenced by increases in home financing costs and their disproportionate weighting in the overall index."

"Recent developments," such as the beginnings of a decline in interest rates, "suggest that this source of upward pressure should be considerably less of an influence over the next several months," Weidenbaum added.

He pointed out that despite recent big rises in consumer prices, "the underlying rate of inflation remains below double digits." A figure that illustrates economy-wide price changes, called the gross national product (GNP) deflator, was released last week showing inflation at an annual rate of 9.4 percent in the third quarter of this year. Producer (wholesale) prices also have shown only moderate increases recently.

Both the CPI and the GNP deflator have clearly taken a turn for the worse in recent months, however. Between March and June this year the deflator rose at an annual rate of only 6.4 percent while the CPI increase was 7.4 percent. In the three months to September, however, the deflator was up by 9.4 percent, but the CPI leaped by 13.5 percent at an annual rate.

As an indication of how the treatment of housing has exaggerated the ordinary CPI, an experimental consumer price index that treats housing differently showed a price rise of 9.2 percent in the year to September, compared with the 11 percent recorded by the ordinary CPI.

Last month's jump in prices helped to lower real earnings for the nation's workers. Inflation-adjusted spendable earnings dropped by 1.9 percent in September after showing no change in August and falling by 0.6 percent in July.

Economists warned earlier this year that some of the early price improvement was due to temporary factors, such as unusually stable food prices and a drop in oil prices. Further gains would be much harder to win and would depend heavily on the big labor contracts to be negotiated next year, they said.

The administration has said that its policy of encouraging tight money, cutting the federal budget and encouraging deregulation will combat inflation. Other analysts believe that wages, which determine the underlying level of inflation, may not respond to this, and that the administration's anti-inflation policy may boil down to a prolonged period of relatively high unemployment and sluggish economy.