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 said the Department of Justice wrongly determined that the 63-year-old Milltown bomber is now eligible to seek freedom from jail.
It follows a legal challenge by Deborah McGuinness, the sister of one of Stone's victims.
Her brother, Thomas McErlean, was among three mourners murdered by Stone in the attack on an IRA funeral at Milltown in west Belfast in March 1988.
Outside court an emotional Ms McGuinness expressed relief and delight at the outcome.
"I just took this case to ensure Michael Stone served his sentence in full.
She continued, "He was given 30 years in custody and I wanted certainty that he will do that."
Mr Justice McCloskey, sitting with Mr Justice Colton on Tuesday 15 January, said a six-year period the ex-UDA man spent out on licence before he launched an attempt to murder Sinn Féin leaders Gerry Adams and Martin McGuinness at Stormont in 2006 should not count towards his 30-year minimum term of imprisonment.
They ruled that the earliest date Stone can be released on licence is July 2024.