A federal judge has declared unconstitutional a Reagan administration policy of barring aid money for private organizations that promote abortion as a form of worldwide family planning.

U.S. District Judge June L. Green ruled that the policy, first announced in 1984 at a United Nations conference on population in Mexico City, violated the free speech and association rights of a U.S.-based organization that promotes family planning projects in Third World nations.The policy, announced by then-Undersecretary of State James L. Buckley, has drawn accusations from Third World nations that the United States is interfering with the family-planning policies of those countries.

In an opinion issued Friday but not made public until Monday, the judge ordered the Agency for International Development to stop enforcing the policy.

But Green said AID could bar foreign organizations that receive U.S. aid from using the money to finance abortion services.

The ruling was made on a 3-year-old lawsuit by three family-planning organizations that operate programs in the Third World, including DKT Memorial Fund Ltd, which is based in New York.

Green ruled that Parivar Seva Santha of New Delhi, and Population Services Family Planning Programmes Ltd., of London, lacked legal standing to press their constitutional claims because they are foreign organizations.

But she said the regulations "impair the associational rights of DKT" under the First Amemdment.

"AID is in essence coercing the plaintiffs to refrain from abortion-related programs if they wish to receive AID funding, even though they are using their own funds for the abortion-related programs," she added.

AID spokesman Jerry Lipson said the agency had not seen the opinion. But he said AID will "certainly recommend that the Justice Department appeal any aspect of the court's ruling that is unfavorable to the agency and (that) hey do so on an expedited basis."