PORTLAND – An adult adoption involving lesbian partners and a claim to a share of one of America’s premier business fortunes has been annulled, bouncing their high-profile case back to Maine’s highest court.

At issue is whether it was legal for a judge to allow Olive Watson to adopt Patricia Spado in 1991 in Knox County, where the longtime partners spent several weeks each summer on an island in Penobscot Bay.

The relationship ended a year after the adoption was approved. In 2005, heirs to the late Thomas Watson Jr., who built International Business Machines Corp. into a computer giant, challenged the adoption in court.

The probate judge who granted the adoption annulled it on a residency issue on April 24, but her sealed ruling didn’t come to light until an appeal brief was filed with the state supreme court. In Maine, adoption records are confidential. The fact that the women were in their 40s when the adoption took place has no bearing on the confidentiality.

Gay rights activists say the case shows the lengths to which same-sex couples would go to ensure a partner’s financial security in the days before they were allowed to form civil unions or to marry.

But the stakes involved in this dispute are higher than most. They include a share in trust funds that Thomas Watson set up for his grandchildren, who number at least 18, not including Spado. Watson died in 1993, unaware that his daughter had adopted Spado, and his wife died in 2004.

With both parents dead, the grandchildren became eligible for cash payouts at age 35. Spado’s claim that being a granddaughter made her a potential beneficiary was challenged by the family in probate court in Greenwich, Conn., where Thomas Watson lived at the time of his death.

Olive Watson and Spado had been living in New York at the time of the adoption, but that state barred the adoption of a homosexual partner.

Maine law required that the adoptee had to “live” in the state and the adopting parent had to “reside” there. But Maine’s adoption law does not specifically define either term.

During their 14 years together, Spado and Watson spent several weeks each summer at Watson’s home at her family’s compound on North Haven, known as Oak Hill. In pursuing the adoption in Knox County, Watson was guided by advice from a Maine lawyer.

In their legal brief, Spado’s attorneys argue that the enormity of the annulment of a 17-year-old adoption on the basis of such undefined domicile requirements after the challenge by Watson family trustees cannot be overstated.

“Most disturbing, this challenge can come not just in a direct appeal, but at any time, even decades later, in a collateral attack long after final judgment and longstanding reliance on the adoption,” the brief stated.

Also appealing the case are trustees of the family trusts, whose attorney expects to file his brief at the end of the month.

Rockland attorney Stephen Hanscom plans to argue that the adoption also should have been annulled on other grounds: that it was obtained by two partners seeking to manufacture inheritance rights and that they did not intend to establish a normal parent-child relationship.

Also at issue in Spado’s appeal is her claim that Knox County Probate Judge Carol Emery should have withdrawn from the case because of the possibility she would be called on to testify about whether she was made aware of the women’s sexual relationship when she approved the adoption.

A judge in Connecticut has ruled that Thomas Watson did not recognize Spado as his granddaughter and did not intend for her to benefit from his trusts. Spado has appealed that ruling, but the Connecticut courts are awaiting Maine’s ruling because the case becomes moot if the adoption is annulled.

The Maine Supreme Judicial Court is likely to hear arguments on the appeals sometime this fall.

Spado and Olive Watson had lived together for 14 years before their breakup, moving 11 times but spending only five nights apart. Under their separation agreement, Watson paid her ex-partner about $500,000 in exchange for relinquishing her claim to certain real estate.

The settlement, however, was apparently not intended to terminate Spado’s rights to inheritance as a granddaughter. Her court filings contain a letter signed by Watson after the breakup in which she says, “I shall at no time initiate any action to revoke or annul my adoption of you.”