Italy's rebelious Communist Party has been barred from delivering an address to the Soviet Communist Party Congress unless it drops criticism of the Afghanistan invasion, Italian sources disclosed tonight.

The sources, representing Western Europe's largest nonruling communist party, said they have refused to alter the report. The Italian Communists, whose relations with Moscow have been strained since the 1968 Soviet invasion of Czechoslovakia, said the report also ran into sharp Soviet objections because it supports Poland's independent worker's movement.

The report of disputes at the 26th congress came as Soviet President Leonid Brezhnev met with President Fidel Castro of Cuba today to pledge Soviet backing in the face of an "anti-Cuban" campaign by "U.S. imperialists."

According to the sources, the Italian Communist delegates who waited for four days for the Soviets to back down have given up almost all hope they can deliver the uncensored report in the Palace of Congress where 5,000 Soviet Delegates and more than 100 foreign delegations are in session.

The Italians were reported to have "refused to change a word of the report," making it virtually certain in their view that they will be allowed only to deliver the address in a speech in some obscure setting, such as a factory or school, instead of before the assembled leaders of the world communist movement.

If this occurs, it will be the first time ever that the Italian communist delegation leader had been prevented from addressing the congress, which meets every five years and is the major stage for Moscow's insistence that the movement is unified and intact, despite its quarrels with arch rival China and Albania.

The sources said the Italians have not yet been informed where or when their report will be given, although they say they have been told it will be reprinted in full in the official congress record and published in central Soviet party newspapers.

This method of handling the Italians underscores the importance the Soviets attach to the setting of the congress, where criticism of Soviet policies, and implicity of Brezhnev, cannot be tolerated. This is especially true at a time when Moscow's external relations have been damaged by the Afghan invasion and worries about possible intervention in Poland.

The Soviets were infuriated two weeks ago when Italian Communist Party Leader Enrico Berlinguer decided not to attend the congress and sent deputy Giancarlo Pajetta instead. Berlinguer headed the delegation at the 1976 parley, drawing a cold reception when he spoke of Italian communist autonomy from Moscow.

The ideological struggle between Moscow and the divergent voices of Eurocommunism was more sharply defined then than now, especially since the French Communists have swung again toward Moscow and the Spanish party is under internal pressure to do the same.