Retail sales fell 0.8 percent in June, the second monthly decline in a row, while producer prices for finished goods were unchanged from May, the government reported yesterday.

The two reports provided new evidence that the economic expansion is continuing to sputter, economists said. Earlier figures on employment and hours worked in June also suggested weak growth and little if any increase in industrial production.

Indications that the economy is weakening were cited by foreign exchange dealers yesterday for a sharp drop in the value of the U.S. dollar against other currencies. The report on producer prices and retail sales reinforced apprehensions of a slowdown, dealers said.

Some analysts attributed the retail sales decline, which was larger than expected, to wavering consumer confidence in the economic outlook. Surveys of consumer sentiment show it remaining at relatively high levels but dropping.

Sales for May were revised upward slightly so that the decline from April was estimated at 0.5 percent rather than the 0.8 percent originally reported. Despite the two declines, a large gain in April left sales for the entire second quarter 2.7 percent higher than in the first quarter and 6 percent higher than in the same three months in 1984.

The June drop, to a level of $113.9 billion on a seasonally adjusted basis, came entirely in durable goods, sales of which fell more than $900 million to $41.6 billion. About $400 million of the decline occurred at automotive dealers' stores as car sales fell, particularly at dealerships handling domestic makes.

An end to a number of sales incentives the previous month may have affected auto sales, analysts said, but more fundamental forces may be at work, too.

"Consumer spending, which has been a major driving force for the economy so far this year, is expected to slow from a 4.7 percent rate of growth in the second quarter to a 2.3 percent pace in the third quarter," said economist Allen Sinai of Shearson Lehman Brothers in his latest forecast. "Less growth in real income, an increasingly burdensome debt load, and an end to the accelerated tax refunds all indicate weaker consumption."

The report on producer prices from the Labor Department underscored the easy availability of goods, especially in the industrial sector. The index for crude materials for further processing, such as corn, raw cotton, scrap metals and timber, fell 0.6 percent, the seventh consecutively monthly decline. That index now stands 8.3 percent lower than in June 1984.

Similarly, the index for intermediate materials, which was unchanged in June, is 0.4 percent lower than it was a year ago.

The index for finished goods, also unchanged last month, is up 1.1 percent over the year. Prices of finished consumer goods fell 0.1 percent in June, as both consumer foods and other goods prices fell by that same amount.

Finished energy goods prices, which shot up 3.4 percent in May, dropped 2 percent in June. Those and other energy prices in the various producer price indexes are included with a one-month lag. Beginning next month, the lag will be eliminated, the Labor Department said.

Capital equipment prices rose 0.4 percent in June following two months of no changes. That index is up 2.3 percent from June 1984.

White House spokesman Larry Speakes said the producer price figures "should mean low prices for American consumers in the months ahead. They show the Reagan administration is continuing to hold the line on inflation and doing well in that effort."

The White House issued no statement on the decline in retail sales.