The largest one-month food price increase in five years sent the consumer price index rising again last month after a respite in July, the Labor Department said yesterday.

But the food price boosts were offset by housing and energy costs, which were relatively stable, and the index rose only a relatively moderate 0.7 percent, the department reported.

The department announced separately that grocery prices in the Washington area jumped 2.2 percent in August after an increase of 2 percent in July. Local prices of other goods will not be compiled until next month.

And Howard W. Hjort, the Agriculture Department's chief economist, warned that grocery prices probably have posted another "significant" rise this month, which will show up in the final price report to be published before the November presidential elections.

Perhaps mindful of that prospect, President Carter did not tout the new figures yesterday in a campaign swing on the West Coast. A White House official said only that inflation "remains the No. 1 problem."

Nevertheless, R. Robert Russell, director of the Council on Wage and Price Stabilitiy, told Congress that the August increase in the consumer price index, which amounts to an annual rate of 8.7 percent, is a visible improvement from earlier months.

Russell said he believes the so-called "underlying" rate of inflation, excluding volatile items such as food prices and home mortgage rates, is now between 7 percent and 8 percent compared with 8 percent and 10 percent earlier this year.

Consumer prices speeded up to an annual rate of 18.2 percent in January and February before finally easing in the face of the past few months' recession. Most economists place the underlying inflation rate at between 8 percent and 9 percent.

The August increase followed a respite in July in which the consumer price index actually stayed level for the first time in 13 years -- a performance economists were quick to warn was almost certainly a fluke

The stability in July stemmed primarily from the sharp, one-time decline in home mortgage interest rates, which had lifted the price index artificially last winter and spring.

The decline in home mortgage rates continued to hold the price index down in August. Housing costs over the month rose a scant 0.1 percent.

However, Russell cautioned that with mortgage rates now back on the rise, there would be only about a month or two more in which mortgage rates would "help" keep the index low. There is a two-month delay before rates show up in the index.

The increase in grocery prices last month was the sharpet since 1975. Spurred by huge increases in farm prices in July, supermarket prices soared by 2.3 percent in August, almost doubled the previous month's rise.

The Labor Department added a new wrinkle to its price-reporting efforts yesterday. It unveiled a new compilation of average retail grocery prices, gathered from 2,300 supermarkets around the nation.

The listing showed the average price of choice T-bone steak at $3.85 1/2 a pound in August, up from $3.75 1/2 in July, while white bread fell to 50.7 percents from 51.1 cents the previous month.

Among the other items included were milk, $1.06 a half-gallon, up from $1.05 in July; grade A eggs, 90.7 cents a dozen, up from 77.6 cents; and bacon, $1.47 a pound, from $1.37.

Yesterday's price report also showed these other developments:

Energy. Energy prices edged up 0.2 percent during August compared with a 0.3 percent rise in July, as gasoline prices declined for the fourth consecutive month and utility costs rose more moderately.

Clothing. Apparel prices rose 0.6 percent over the month compared with a 0.4 percent increase in July, mainly because of the introduction of new lines of fall and winter clothing.

Transportation. Transportaion costs surged 0.9 percent in August, the largest monthly rise since March. New-car and used-car prices were the primary factor in the increase. However, public transportation costs jumped 4.4 percent.

Medical services. The cost of medical care rose 0.7 percent in August, the same as in July. Hospital charges surged by 1.5 percent, while physicians' fees rose 0.5 percent.

The August increase brought the overall consumer price index to 249.4 percent of its 1967 average, meaning it took $249.40 to buy the same goods and services last month that cost $100 just 13 years ago.

The relatively moderate rise in August kept workers' purchasing power from eroding further. The department's index of hourly earnings, adjusted for inflation, remained essentially unchanged at 3.3 percent below a year ago.