Soviet authorities have reinstituted a system of informal food rationing here in a precautionary move to prevent food shortages expected as a result of this year's poor harvest.

Well-informed sources said Communist Party members have been told in closed meetings that urgent and discreet measures will have to be taken to preserve the available food supplies.

Grain, dairy products and potatoes appear to have been the items most sharply reduced by the unseasonably hot and dry summer. This in turn has again led to meat shortages.

The grain harvest is now expected to yield no more than last year's disappointing 190 million tons -- the third year in a row the Soviet harvest will fall 40 to 50 million tons short of the 1978 harvest of 236 million tons.

The precautionary austerity measures are also believed to be linked to Soviet food shipments to Poland. This has not been mentioned publicly here. But the Polish newspaper Trybuna Ludu reported a week ago that the Soviets were sending frozen fish, canned goods, cooking oil and other commodities. And Polish officials said these emergency shipments also include substantial quantities of grain.

According to the sources, a Soviet Central Committee letter read to local party organizations called for the strict implementation of a rationing system that has long been disregarded in practice. It allows a person to buy no more than four pounds of meat or a chicken, four pounds of bread, one pound of butter and one pound of cheese during one shopping trip.

Last Wednesday, managers of retail stores in Moscow told their personnel that these regulations must be observed strictly and that violators would suffer severe penalties.

There are no visible indications in Moscow of food scarcities except for meat. The supplies at the city's farmers' market -- where farmers sell the produce and meat from their legal, private small plots -- has been very good all summer.

In provincial centers, however, the situation is reported to be much worse than in Moscow and Leningrad, both of which receive special priority for consumer goods. Westerners returning from trips to smaller cities have reported an almost complete absence of meat.

Some Soviet citizens said the new rationing on the basic commodities was directed against tens of thousands of people from the provinces who flock to the main cities each weekend to buy large quantities of food.

But Western observers here believe that the poor harvest and the continued Polish crisis make it imperative for the Kremlin to begin husbanding its food resources well in advance of what is expected to be another period of inevitable scarcities this winter.

The only clear public indication of the new policy was contained in a decision of the Presidium of the Supreme Soviet last week. It called on regional authorities to "assume control" over the production and sale of food and the way it is used.

The presidium also called for a propaganda drive to instruct people in the "efficient use of bread and food products."

One such effort has been mounted at Bakery No. 17 in downtown Moscow. It has organized public demonstrations on the "50 ways to make tasty dishes" from old bread.

One Western agricultural specialist said that three straight years of bad harvests coupled with the now-lifted partial U.S. grain embargo have made Moscow's situation quite uncomfortable. He added that the apparently open-ended commitment to Poland could turn it into a painful affair.