The collapse of the Christian Democratic minority government of Prime Minister Giulio Andreotti became inevitable today when leaders of four parties that have backed it for the past 18 months formally withdrew their support.

Following a meeting with parliamentary leaders, Andreotti said he would submit his resignation on Monday - a move likely to inaugurate a prolonged political crisis and possibly lead to new general elections.

The Christian Democratic leader had been seeking a way of keeping his minority government in power without direct Communist participation.

If Andreotti's resignation is accepted, it appeared likley that he would be asked by President Giovanni Leone to try to form a new government - Italy's 40th since the fall of fascism 33 years ago.

Andreotti said after today's talks with parliamentary leaders that it was clear that several of the parties that had kept his government alive now want to "have an in-depth, rediscussion of a program and support for the government."

Demands for the end of Andreotti's government were triggered last November by two of the six parties that have been propping the minority Christian Democratic Cabinet. But the real decisive move came this week when the Communists, the nation's second largest party and a part of the six-party parliamentary coalition, joined the effort to topple the government.

The Communists, who control 221 votes in the 630-member Chamber of Deputies, have demanded for the past four weeks that Andreotti form an emergency coalition cabinet in return for its continued support. Communists would share ministerial positions in such a government and direct governmental responsibilities for the first time in 31 years.

The Christian Democrats, Italy's largest party, have turned down the Communist request. The party said it could not go beyond strengthing of a six-party program worked out last July that formalized the Communists" influence without giving them governmental responsibilities.

Apart from Italy's domestic instability, the prospective political crisis raises a larger issue of increasing Communist influence in Western Europe.

The possibility of the Communists gaining decision-making roles in a NATO country have produced alarm in Washington. And coming only two months before elections in neighboring France, where the left is expected to do well, the current instability here had prompted the Carter administration to issue a stern warning against any Communist role in allied governments.

The Communists responded yesterday by describing the U.S. statement an "open and coarse interference in Italian affairs" and by calling a nationwide campaign of demonstrations and public meetings to back the Communist drive for a governmental role.

Andreotti's foreign minister, Arnaldo Forlani, implicity criticized the American statement last night by saying, "These things have never been any help in sorting out our complicated situation.

"There is nothing new in the U.S. statement from what they have said in the past. Therefore one could do without asking them what their position is."

Political sources said Andreotti was expected to court left-wing support for a new government bsed on wider parliamentary cooperation on key issues such as Italy's flagging economy and extremist violence, which has already claimed three lives this years.

Such support most likely will be patterned on the July agreement between the Christian Democrats, the Communists and four other parties in which the Christian Democrats in effect acknowledged their inability to pass unpopular measures without leftwing support.

The Communists enjoy broad support from Italy's powerful labor union and a growing number of Italian politicians now believe that Communist help is needed to engineer a crucial cost-cutting "social pact" with the unions as well as the reduced social services that would result from budget cuts called for by the International Monetary Fund as a prerequisite to continued international aid.

But the Communist question remains at the center of Italian political life for another reason. Since the elections of June 1976 the Christian Democrats have been unable to win sufficient support among Italy's smaller parties to form a new center-left majority similar to those that governed Italy from the early 1960s through 1976.

Since the spring of 1976 Italy's Socialists have refused to join any government excluding their Communist cousins. During the same period the Socialists have repeatedly sought to embarrass the Communists by claiming that they, and not the Communists, are this country's real leftists. High-placed Communist sources have confirmed that the Socialists' ideological sniping was one factor leading to the hard-line position adopted last December.

The Communists also explain the turnabout by pointing to government delays in implementing parts of the July program but according to one member of the Communist Executive committee that this is only a minor reason for the party's switch.

Since September complex economic problems requiring delicate negotiations between parties and spiraling terrorism and politically motivated violence have slowed down government activity. An unfulfilled commitment to the IMF for drastic budget reductions also recently led to bitter criticism.

But among its achievements the Andreotti Cabinet can point to a substantial cut in a high inflation rate and an impressive improvement in the country's financial situation.

New statistics show that the country's once-troubled balance of payments is now running a surplus for the first time since 1971 and that foreign currency reserves have soared to $8 billion after hitting a low of $1 billion only two years ago.

The stiffening of the Communist position thus primarily reflects concern over the party's current image with its traditional electorate, largely union members who in recent months have become increasingly dissatisfied with the Andreotti government.

Some observers here speculate that under Soviet pressure the Communists have decided to end their compromise with Italy's Catholics and return to their traditional opposition role.

These observers point to French Communist leader George Marchais' recent break with his Socialist allies. But there is no hard evidence to support such a theory and generally reliable Communist sources insist their major goal is greater control over a more efficient government that will enable them to persuade rank and file members that the compromise with the Catholic Church is paying off. Significant local elections are scheduled for May and the Communists do not want to lose ground.

Andreotti is understood to favor a compromise by which the Communists would renounce demands for government posts in exchange for formal acceptance into the parliamentary majority on which the government rests.

The Communists are said also to want a say in the selection of new Cabinet ministers and periodic consultations to monitor government performance, but politicians in both the Christian Democratic and Communist parties seem to feel there is room for agreement. The alternative would be new general elections, which would probably be most damaging to the small parties like the Socialists and the Republicans.

Elections would increase hostility between Italy's two major parties, are thought unlikely to alter the basic division of power here, and would leave Italy - with it myriad economic and social problems - without a government at the time of gathering recession and growing political violence.

Those who want a vote are fearful that if the Communists get into the majority now, they could trigger a Cabinet reshuffle sometime after June when, because of next December's presidential election, a six-month constitutional ban on elections goes into effect.