The state’s highest court ­ordered Thursday that an affidavit detailing a minor’s allegations that she was raped by Quincy developer William O’Connell at his residence at Quincy Bay Marina be made public.

In a unanimous ruling, the Supreme Judicial Court backed a request by the Patriot Ledger newspaper to unseal the search warrant affidavit filed in ­Quincy District Court by State Police Trooper Kathleen Prince last year as prosecutors investigated rape allegations against O’Connell.

The documents were first sealed, but a district court judge later agreed with the newspaper and ordered its ­release, prompting O’Connell and the special Norfolk prosecutor on the case to ask the SJC to keep the record sealed. O’Connell argued that his right to a fair trial was threatened by the disclosure, and prosecutors contended that the record must be kept confidential under a state law making police reports on sexual crimes confidential.

O’Connell’s attorney, ­Stephen R. Delinsky, said that while he respects the court, he was “very disappointed’’ by the SJC’s ruling because he now fears that an impartial jury will be hard to find.

“It will make it much harder for Mr. O’Connell to receive a fair trial,” Delinsky said. “It will complicate jury selection greatly. We will do everything in our power to make sure he gets a fair trial.”

In a friend-of-the-court brief filed with the SJC, Attorney General Martha Coakley urged the SJC to keep the document impounded out of concern that victims of sexual crimes will not cooperate with law enforcement if they know details of the incident will be made available to the public.

In a statement, Coakley said she is concerned that the court has made it harder for law enforce­ment to gain and keep the trust of victims of sexual crimes.

“It is vital for victims of sexual assault to be encouraged to come forward so that they can seek safety and assistance and so that offenders can be held account­able,’’ she said. “We are concerned that this decision will deter victims from reporting.”

In its ruling, the SJC concluded that the state law keeping police reports of sexual crimes confidential does not ­apply to court records, especially affidavits prepared by police as they seek judicial approval for a search warrant.

Adopting the proposal backed by O’Connell and prosecutors would impair the public’s right to observe the criminal justice system, Justice ­Fernande R.V. Duffly wrote for the court.

“If applied to judicial ­records, such a requirement would have unacceptably far-reaching consequences,” Duffly wrote. “A wide range of court records in cases involving allegations of rape or sexual assault would be subject to mandatory and permanent impoundment, regardless of the specific facts or circumstances of each case. The United States Supreme Court has emphatically rejected blanket prohibitions on public access to judicial proceedings.”

The SJC also said that when O’Connell goes to trial, the judge in charge of his case will have wide-ranging powers to ensure that potential jurors are not influenced by news coverage of the case and to take other steps considered necessary to protect O’Connell’s Sixth Amendment fair trial rights.

“In the context of a criminal proceeding, the public’s right of access, whether derived from the United States Constitution or common law, must be balanced against the defendant’s constitutional right to a fair ­trial,” Duffly wrote. “Judges are well equipped to safeguard a defendant’s right to a fair trial.”

Last September, O’Connell pleaded not guilty in Norfolk Superior Court to 12 charges, including four counts of aggravated statutory rape, two counts of engaging in unnatural acts, and one count of trafficking in cocaine. He is free on $150,000 cash bail.

Coakley’s office appeared ­before the SJC to argue about the legal issues raised in the O’Connell case.

O’Connell is being prosecuted by special prosecutor ­Andrew DiCarlo Berman at the request of Norfolk District ­Attorney Michael Morrissey to avoid a conflict of interest. While he was a lawmaker on Beacon Hill, Morrissey worked with O’Connell on development projects.