Italy's Communist Party today withdrew its support from Premier Giulio Andreotti's minority government, setting off a new government crisis and breaking its 2 1/2-year-old uneasy alliance with the ruling Christian Democrats.

The action doomed the 11-month-old Andreotti government and raised a distinct possibility of early parliamentary elections.

The policy shift was apparently precipitated by continued Communist inability to gain a share of Cabinet positions despite their support of the Christian Democratic government.

The Communist-Christian Democratic cooperation has been widely regarded as an outgrowth of the switch to the "demociatic path of power" adopted by the Eurocommunists of Western Europe. The Italian Communists were the champions of this movement.

Today's move is thought likely to herald a return to the policy of straightforward opposition the Communists followed until 1976, when they agreed to give an earlier Andreotti Cabinet crucial parliamentary support through abstention on key votes of confidence.

"We have reached the conclusion," Communist leader Enrico Berlinguer said, "that our permanence in the majority that supports the government has become impossible."

He called for a national unity government including Communists in the Cabinet, a bid the Christian Democrats immediately termed "not realistic."

The Social Democrats -- who with the Communists and three other parties had jointly supported the Andreotti Cabinet -- also announced today that they would vote against the government during a parliamentary session on emonday.

Andreotti is expected to submit his resignation to President Sandro Pertini on Tuesday.

Pertini is expected to ask Andreotti to form another Cabinet. This will be followed by weeks of complex bargaining at a time of growing unemployment and urban terrorism in the country.

The Communist decision to seek confrontation with the noncommunist parties is believed to be linked to the forthcoming party congress in March, its first in four years. It was given added impetus by a speech Christian Democratic leader Benigno Zaccagnini made recently in the United States in which he asserted that a Communist would never join the Cabinet.

But blocking of repeated Communist attempts to get a share in the government is only one reason for the current crisis.

In reality, the breakdown in Communist-Christian Democratic relations has been brewing for months. The Communists have accused their de facto partners of backtracking on the policy of close teamwork mapped out by former Christian Democratic premier Aldo Moro before his kidnaping and murder by leftist terrorists last spring.

But the Communist decision to bring down the Andreotti government that was formed last March 16, only hours after Moro had been kidnaped, primarily reflects the growing conviction of most of the party's leaders that the party's support for a Christian Democratic government simply was not paying off.

"We sold our support too cheaply," said a veteran Communist who explained that grass roots confusion and a series of recent poor election showings indicated that the party's "step-by-step" policy of approaching power had backfired and needed revision.

Last weekend at a party festival in Folgaria, a crowd of more than 300 local Communists roared with approval when Politburo member Luca Pavolini told them the party would no longer "play the water boy" for a government "which has failed to live up to its commitment."

But Pavolini, presiding at the festival whose sponsors included Coca-Cola, took great pains to justify the party's course so far. He stressed that there would be no backing away from Eurocommunist principles.

Today's pullout from the five-party alliance marks the second time in 30 months that the Communists have forced Andreotti to resign.

Just a year ago they precipitated another crisis, arguing that they be given a policy-making role commensurate with the 34.5 percent of the vote polled by Communists in the 1976 elections.

After two months of bargaining, the Communists won an unprecedented formal role in what Italians call the legislative majority, the parliamentary party alliance from which a Cabinet draws its support. This gave them a say in the government's program and brought them closer to formal power than at any time since 1947, when they were forced out of the postwar coalition government.

The new crisis, the 40th since the fall of fascism in 1943, comes at a time when a much-needed economic package was about to go before parliament.

What happens in the next few weeks is anyone's guess. Inside observers point out that the attitude of the 82-year-old president could be crucial. Pertini, elected to the country's highest office last July, is known to be opposed to the idea of early elections, which are otherwise scheduled for the spring of 1981.

Before dissolving parliament he is likely to try to persuade the parties to accept an alternative solution. This could be a nonpolitical government of technocrats, a "centrist" government of Christian Democrats and several smailer parties or -- if his own Socialist Party could be persuaded to go along -- a new center-left government of Christian Democrats and Socialists like that which governed Italy from 1962 to 1975.

Many Communists have said privately that they do not want elections either, preferring a lengthy period of opposition in which they can recreate the old "fighting" image that in 1976 won them an unprecedented 12 million votes.

But, they say, they will run the risk of a vote. According to recent polls, the Communists would lose ground while the Christian Democrats would gain.

"Other Italian parties act in their own interest. When we do so we are accused of being irresponsible," a high-ranking Communist said.