In a significant modification of its "hands-off" policy toward Communist parties in Western Europe, the Carter adminstration decided yesterday to express public concern at the prospect of Communist participation in Italy if the hard-pressed minority Christian Democrat government there falls.

President Carter, after a series of meetings throughout the day with top State Department and national security officials, reportedly agreed to go public after hearing a report on the wear-crisis in Italy from Ricahrd K. Gardner, the U.S. ambassador to Rome, who was recalled for consultations.

Officials said Gardner urged a firm U.S. respone to the propect of a Communist role in an Italian government, if only to strenghten the hand of Christian Democratic politicans against such a development.

The amdassador, a former university professor, was chosen for the post early in the administration in part as a signal to Europeans that a dogmatic U.S. opposition to "Europecommunists" was a thing of the past. He is understood to have warned Carter that Italian politicans were not taking seriously expressions of U.S. concern, now that Communist entry into the government has become a live possible ty.

At the very least, officials said Garner wanted to return from Washington with presidential authority to assert forceful but private opposition to Communist participation in future conversations with Christian Democratic leaers.

It is understood the statement prepared for public release today is cautiously worded, asserting specific concern at Italian political developments and at the prospect of participation by a "non-Democratic" party. At the same time, it is expected to reassert the Carter administration formula that "the position of a communist party in a particular country is a matter to be decided by the government and the people of the country concerned."

The statement prepared yesterday for realese at the State Department, was cleared at a White House meeting at which key participants were Carter's national security adviser, Zbigniew Brzezinski; Deputy Secretayr of State Waren W. Christopher; Gardner and Assistant Secretary of State George S. Vest.

United Press INternational reprted from Rome that Premier Giulio Andreotti's Christian Democratic Party voted last night against giving cabinet seats to the Communist Party, and leftlists threatened to unite to topple the government.

The Christian Democratic Directorate voted unanimously to approve a report by Party Secretary Benigno Zaccagnini which said the party could not go beyond a 1975 agreement under which Communists give indirect parliamentary support to Andreotti's minority government.