Loyalist killer Michael Stone must serve a further five-and-a-half years in prison before he can be considered for release, the High Court has ruled.
Judges held that the Department of Justice (DoJ) had wrongly determined he would now be eligible to seek parole.
In December, the sister of one of his victims won permission to challenge his eligibility to do so.
Deborah McGuinness was granted leave to apply for a judicial review of the DoJ's calculations of his time served.
Her brother, Thomas McErlean, was among three people killed by Stone in a gun and grenade attack at Milltown Cemetery in Belfast.
The judges' verdict, on Tuesday, came as his case was set to go before the parole commissioners.
That hearing has now been put on hold pending any appeal against the ruling.
Stone, 63, was freed under the terms of the Good Friday Agreement in 2000, but was returned to jail six years later for trying to kill Sinn Féin leaders Gerry Adams and Martin McGuinness at Stormont in 2006.
Mr Justice McCloskey, sitting with Mr Justice Colton, said a six year period spent out on licence before the murder attempt should not count towards his 30 year minimum term of imprisonment.
He forfeited the benefits of that exceptional release by opting to commit further serious terrorist offences, the judges pointed out.
Quashing the DoJ's decision, Mr Justice McCloskey confirmed: "The earliest date upon which Mr Stone might be released on parole licence will fall around July 2024."
Ms McGuinness said she was relieved with the outcome.
"I just took this case to ensure Michael Stone served his sentence in full," she said.
"He was given 30 years in custody and I wanted certainty that he will do that," she added.
Mr McErlean died in an attack on an IRA funeral in Milltown Cemetery in 1988.
The ex-Ulster Defence Association (UDA) member was also the gunman in another three separate killings:
- Milkman Patrick Brady was murdered in south Belfast in November 1984
- Joiner Kevin McPolin was shot in the head in Lisburn, County Antrim, 12 months after Mr Brady
- In May 1987 Dermott Hackett, a bread server, was found dead in his van between Drumquin and Omagh. He had been shot up to 16 times with a submachine gun.
Mr Justice McCloskey said: "The Belfast Agreement afforded to convicted terrorists such as Mr Stone the opportunity to demonstrate that they were worthy of the exceptional benefit of significantly accelerated release from sentenced custody.
"Those, such as Mr Stone, who have elected not to do so forfeit this benefit and revert to their pre-release status."
"The calculation of Mr Stone's earliest release date must make no allowance for the period of approximately six years which elapsed between the date of his exceptional early release from prison and the date of his subsequent further detention."