The family of Helen McCourt are seeking an urgent judicial review to prevent her killer being released from prison despite never revealing where he hid her body.
A legal team has made the request on the Billinge family's behalf, her brother Mike confirmed, after the Parole Board refused the Justice Secretary's request to keep murderer Ian Simms behind bars, standing by its original decision to free him.
Ms McCourt's mother Marie had called on Justice Secretary Robert Buckland to step in after the board decided Simms had met the test for release three decades after the 22-year-old insurance clerk went missing.
Mike McCourt said: "We are seeking an urgent judicial review. That is our next step. We have a legal team helping us out."
He said lawyers had already been in contact with Mr Buckland about the move.
Describing the family's reaction to Wednesday's decision by the Parole Board, Mr McCourt said: "My mum took it in her stride.
"I expected the decision to go that way. I hoped, but I knew they wouldn't overturn their own decision.
"When it came out it was hard to take. It all seems to be going against us but we have to keep fighting."
On Wednesday Marie McCourt and Mr Buckland both said they were "disappointed" with the decision.
The minister's request for a reconsideration was made on grounds including that the original decision was "irrational" and "erroneous", according to documents on the application.
In his findings, Parole Board chairman Sir David Calvert-Smith, a former director of public prosecutions and retired judge who reviewed the request, said it was "impossible" to characterise the decision as "irrational" so the application was refused.
Simms will not be released immediately and it could take weeks to arrange and may be dependent on whether campaigners launch legal action against the ruling.
The original decision came after Mrs McCourt's campaign to keep killers behind bars until they lead police to a victim's body, dubbed Helen's Law, failed to be ratified before Parliament on numerous occasions - twice being delayed because of general elections.
Mr McCourt said it was "ironic" that the Prisoners (Disclosure of Information about Victims) Bill named after his sister was brought back to Parliament on the day the Parole Board's decision was announced.
A date has not been set for the Bill to be properly debated by MPs.
Pub landlord Simms, who was convicted by a jury on overwhelming DNA evidence of Ms McCourt's abduction and murder, is serving his life sentence at HMP Garth in Leyland.
He has always maintained his innocence over the death of Ms McCourt, an insurance clerk who vanished on her way home from work in Liverpool in 1988.
Simms was convicted of her murder the following year, having been told he would serve at least 16 years and one day behind bars.
He was eligible to be considered for release in February 2004.
The summary of the Parole Board's original ruling said Simms was deemed suitable for release due to factors including the "considerable change in his behaviour".
The decision to release him was subject to a number of conditions including residing at a designated address, having to wear a tagging device to monitor his whereabouts, observing a curfew and avoiding contact with the family of his victim.