An elderly Brooklyn woman’s body was left to rot at a rehab facility when the funeral home tasked with handling her arrangements allegedly failed to immediately retrieve the remains, according to a lawsuit.

Jean Bryan, 86, died in September after a stint at Upper East Side Rehabilitation Center, where she’d gone after falling and hitting her head in her Williamsburg home three months earlier, her family said in court papers.

Bryan, who wanted an open-casket service and to be interred in a grave, had preplanned her funeral arrangements with Lawrence H. Woodward Funeral Home in Stuyvesant Heights, according to the legal filing. When she died, her niece, Xiomara Nichols, promptly called Woodward.

But no one called back, claims Nichols, who said in legal papers that her beloved aunt’s body sat in the rehab center without being refrigerated for two to three days before it was finally taken to the city morgue.

The funeral home didn’t respond to Nichols messages until seven days after Bryan’s death, when it declared her body “was so decomposed it was unviewable,” according to the lawsuit.

“When I saw her body, it was decomposed — I lost it,” a distraught Nichols told The Post. “No one could have ever prepared me to see what I saw.”

The long retired Bryan, who had worked on Wall Street handling accounting for a large firm, was eventually cremated and a service finally held Oct. 1, nearly a month after her death, Nichols said in the lawsuit.

“She was a very dignified woman, very proud, strong and a beautiful woman,” Nichols recalled. “She showed me everything she wanted to have when she passed on. She picked out the dress, the gloves, everything.”

“She could not be embalmed. At that point her body was too decomposed,” said Nichols’ attorney, Robin Gray.

A manager for Woodward insisted the funeral home had no record of prearranged services for Bryan, and needed signed paperwork from the family to retrieve her body.

“Our removal service informed the nursing home that we had no permission to remove the remains and that they should have her removed to the Office of the Chief Medical examiner for holding. The nursing home failed to do what was asked of them by our removal service. This is a common problem with nursing homes as they do not have proper refrigeration services for their clients,” Woodward manager Lynda Thompson told The Post.

Thompson pinned responsibility for the ghastly situation on Upper East Side Rehabilitation.

“Mrs. Bryan’s funeral director was so angry at the condition of her remains she complained to the state. … Instead of moving her remains to the medical examiner until the funeral home could speak with the family, they held at their establishment,” Thompson said.

A spokesman for Upper East Side Rehabilitation disputed that version of events.

Bryan died around 6:30 p.m. on Sept. 4, and her body was picked up by the city Medical Examiner’s office around 5 a.m. on Sept. 6, said spokesman Michael Balboni, who added the facility is “sorry for the family’s loss” but called the timing of her remains going to the city morgue “customary.”

“Even though she didn’t have children, she was like the best aunt anyone could have,” Nichols said. “I saw her as more than an aunt, I saw her as a mother as well.”

The funeral home and the rehab were each required, under public health law, to contact the city morgue to handle Bryan’s remains if Woodward couldn’t, Nichols alleged in the legal papers.

Nichols is seeking $175,000 damages.