The New South Wales attorney general will not intervene to force the prosecution of a man charged with the murder of toddler Cheryl Grimmer 49 years ago.

Mark Speakman said on Friday that he would not use his power to instruct the department of public prosecutions to appeal the supreme court’s decision to omit a crucial piece of evidence in the cold case.

“Having looked at the judgment very carefully, and having taken the advice of the crown advocate, I have concluded that any appeal will fail [and] therefore I can’t bring an appeal,” Speakman said.

“But even if I could bring an appeal, it would just raise false hope and be cruel.

“I know it is a kick in the guts for the Grimmer family … They’ve searched for justice for almost 50 years but unfortunately I can’t help them in that search for justice.

“I can’t think of any other path for the Grimmer family.”

Cheryl Grimmer, who was born in the UK but moved to Australia with her family, vanished from outside a shower block while with her mother and three older brothers at Fairy Meadow beach in the Illawarra region of New South Wales on 12 January 1970.

The man, known only as Mercury because he was underage at the time, pleaded not guilty in September 2018 to murdering the three-year-old in 1970.

In April last year, the court heard that during an interview with police in the early 1970s, the man had confessed to killing Grimmer, telling officers he had intended to have “sexual intercourse” with the girl before allegedly killing her.

The court heard at the time that the crown’s case “largely depends” on an admission made by the man when he was 17 years old, less than 18 months after Cheryl disappeared.

But in February supreme court Justice Robert Allan Hulme ruled the interview could not be used as evidence in the trial.

In his judgment, Hulme said the interview was inadmissible because of the manner in which it was conducted and the particular vulnerability of the accused at the time.

“No parent, adult or legal practitioner was present at any stage of the police interview,” the judge said.

After the decision, prosecutors dropped the case, saying there was insufficient evidence to proceed without the interview. The director of the DPP, Lloyd Babb, declined to appeal the decision.

After appeals from the Grimmer family, who still live in NSW, Speakman sought independent advice from the crown advocate, David Kell, before also concluding that any further action would fail.

He said he had spoken to the Grimmer family on Friday morning and that they were “devastated” by the decision.

“They had lived in hope that at long last, after almost half a century, this crime would be solved and a perpetrator brought to justice,” he said.

Hulme ruled that when Mercury made his admission as a 17-year-old in 1971 he was without an adult or guardian, was of below-average intelligence, immature and more vulnerable than a typical person his age.

While there was no legal statute at the time requiring an adult to be present during the interview of a minor, that law has since been changed and Hulme ruled that it should be applied retrospectively.

Speakman said it was “crystal clear” the law should be applied retrospectively.

“I wish I could rewrite history, I wish circumstances of that interview in 1971 were different [and] I know this decision is a kick in the guts to the Grimmer family,” he said.