The Maryland Court of Appeals today found unconstitutional a key section of the state's child adoption law, holding that it amounts to a denial of due process for the natural parents.

Court-ordered placement of children in foster homes for two years or more is not an adequate basis, by itself, to take the children from their parents and open the door to adoption, the judges ruled.

The action upheld a Washington County Circuit judge's finding in a case involving a 30-year-old unemployed mother of five who was allegedly abandoned by her husband.

Elizabeth Renuart, a Legal Aid Bureau attorney who represented the woman, said the ruling appeared to be a "significant" victory. Opposing lawyers from the state Attorney General's office have said that such a ruling could affect a substantial number of pending domestic custody cases in the state.

The provision thrown out by the state's highest court today allowed trial judges to "presume" that it is "in the child's best interest" to grant guardianship to county social service agencies--the first step toward adoption--when a child has been in a foster home for two or more years.

Judges order children placed in foster homes in cases of physical abuse, nonsupport or other forms of neglect. In its unanimous ruling, the Court of Appeals noted that state law generally requires "clear and convincing evidence" that the natural parents' right to the child should be terminated before guardianship is granted to a social services agency.

But in cases where a child has been in a foster home for a long time, the court said, "the requirement of 'clear and convincing evidence' has been eliminated" and cannot be overcome except by contrary evidence produced by the parents.

In the Washington County case, Mary Alice Clark claimed that her husband deserted her and their four small children in 1979 while she was pregnant. Unemployed with no job skills, driver's license or access to either telephone or bus service in her rural home near Hagerstown, she sought help from the county Social Services Department.

Among other actions, her five children were removed from the home in 1980 and placed in foster homes around the county on orders of a juvenile-court judge. Clark contended that while it was difficult to support her children, she maintained strong, affectionate ties with them and did not want to give them up.

The county social services department, however, petitioned for the children's guardianship in 1982, after the requisite two years, contending that their "best interests" would be served by terminating Clark's right to them and making them available for adoption.

Washington County Circuit Judge Daniel W. Moylan rejected the petition, finding the two-year foster home provision unconstitutional.

The appeals court, in an opinion by Judge James F. Couch Jr., agreed.

"The legislature, while requiring clear and convincing evidence that termination is in the best interests of the child, has provided that such standard shall be presumed when the child has been in foster care for two or more continuous years, thus relieving the state of any further burden of proof," he wrote. "In our view, a parent's right to procedural due process is violated by such presumption."