A New South Wales MP has used parliament to reveal the identity of a man who was previously charged by police over the alleged abduction and murder of UK-born toddler Cheryl Grimmer 55 years ago.

Grimmer 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 under the pseudonym “Mercury”, faced a trial in 2018 after he pleaded not guilty. But it then collapsed after a judge ruled that a 1971 police interview where he confessed to the murder was not admissible.

Mercury made the alleged confession, which was the cornerstone of the crown’s case, when he was 17 years old and still a child under NSW law, and less than 18 months after Grimmer disappeared.

On Thursday, Jeremy Buckingham, a member of NSW Legalise Cannabis party revealed the identity of Mercury while protected under parliamentary privilege.

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Guardian Australia is choosing not to name the man. The man could not legally be publicly identified during the 2018 trial because he was aged 15 at the time of the alleged offending.

Ahead of Buckingham revealing the name, president of the Legislative Council of NSW, Ben Franklin, interrupted him. He asked if Buckingham was about to reveal the name, which is currently required to be suppressed by legal order.

Franklin said: “Can I just make these observations before you proceed under privilege, a member is protected from the consequences which would otherwise result from breaching a legal order of this nature, which could otherwise amount to a contempt of court.”

“However, all members have the obligation to use their privilege responsibly, and this house can take action against members who are judged by the house to have abused privilege. The comity between the parliament and the courts should not be treated lightly, and I ask the member to consider these issues carefully before proceeding.”

The revelation of the man’s name has come after Grimmer’s family gave an extraordinary ultimatum to the man last week – agree to meet with them by midnight Wednesday or Buckingham would reveal his identity in parliament.

They also revealed a dossier which holds the confession the man made when he was 17, according to Buckingham.

Police reopened the case after a 2011 coroner’s report ruled that Grimmer was dead and recommended that police reinvestigate.

Mercury was arrested in March 2018, and was then in his 60s. He was released a year later after the interview was ruled inadmissible.

Without it, the NSW director of public prosecutions found there was insufficient evidence for the case to proceed.

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

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

In his judgment in the NSW supreme court, 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.

He heard evidence from two psychiatrists who agreed that the teenager had a below average intelligence, was immature and more vulnerable than the average 17-year-old.

The court heard the accused had a difficult relationship with his parents, a history of running away from home, moving countries, low intellect and limited education.

The crown submitted that at the time of the interview, there were no mandatory requirements or guidelines for an adult support person to be present when questioning minors.

The other evidence before the judge was made up of contemporaneous records and reports from 1970-71, derived from his interaction with the juvenile justice system.