The family of Sean Brown are "deeply disappointed" by the British government's decision to appeal a court ruling that a public inquiry should be held into his murder, its solicitor has said.
In April, Northern Ireland's Court of Appeal said a public inquiry should be established without further delay.
The UK government announced yesterday that it is set to appeal the ruling to the UK's Supreme Court in London.
Niall Murphy, with KRW Law in Belfast, said the family had trusted that the British government "would do the right thing and would abide by the clear direction of three tiers of our judiciary".
He said five High Court judges "have considered the facts and have delivered consistent and clear adjudications as to the law and the state’s international legal obligations".
"The only way that the British government can discharge those international legal obligations is by way of a public inquiry," he added.
Mr Brown was chair of Bellaghy Wolfe Tones GAA in south Derry and was locking up the gates of his club when he was assaulted, abducted and murdered by a loyalist gang in May 1997.
He was bundled into the boot of his car, taken to Randalstown, Co Antrim, and shot dead.
No one has been convicted of his murder.
In the course of a now discontinued inquest it emerged that state agents were among 25 people linked by intelligence to his murder.
Speaking on RTÉ's Morning Ireland, the family’s solicitor said Northern Ireland Secretary of State Hilary Benn has now chosen to pursue "what can only be described as a vexatious appeal to the Supreme Court in London".
As a result, he said, Mr Brown’s 87-year-old wife, Bridie, must now travel to London.
Mr Murphy said "everybody is of the analysis" that the only legal mechanism is a public inquiry.
He said the Taoiseach, Tánaiste, Northern Ireland’s First Minister and the PSNI’s Chief Constable have been "very clear" about the matter.
Mr Murphy said Mr Brown’s family "are very, very disappointed" by both the decision to appeal to the Supreme Court, but also "how that was done yesterday".
'Poor form'
He also criticised the manner in which he was notified of the UK government's decision.
He said that in less than two hours Mr Benn "had convened a press conference outside Westminster, effectively broadcasting the content of a private letter that he had sent to me to the attention of Bridie Brown".
He described this as "poor form", adding "it’s political spin of what is a very sensitive and emotive issue and that's not acceptable".
But, he said, "when all matters are taken into account, the family will regroup, redouble and will pursue the vindication of their legal entitlements".
He said: "It's clear that the British government don't want a public inquiry into the murder of Sean Brown. Can they be forced into it?"
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Mr Murphy said the UK government is "exhausting their legal train track, they're going to run out of train track someday".
He said: "That may be at the Supreme Court, but the delay that they are engaging in is consistent with the delay that they have engaged in for the last 28 years, and I think we should bear in mind some of the specific facts of this case.
"Bridie has inquired what have they got to hide. She's actually terrified as to what they have to hide.
"She received page after page of wholly redacted black pages and Bridie just wants to know what are they hiding."
Mr Murphy said it known for "a fact" that state agents were involved in the murder as it was noted at an inquest at the High Court.
"So, if these are facts that have already been disclosed openly to us, what are they trying to hide?" he said, adding "that is the terrifying issue".
"If the chief constable has no fear of a public inquiry, what have the British government got to fear from a public inquiry?" he added.
Mr Brown's daughter Siobhan branded the NIO's handling of the announcement yesterday as "disgraceful".
She said she had not even time to inform her mother about the move before Mr Benn appeared on the media announcing his decision at Westminster.
"I hadn't even got the chance to show that letter to my mother whenever Hilary Benn stood in Westminster and gave a press conference," she told BBC Radio Ulster.
She added: "It's totally disgraceful. We were distraught at the fact that this information had been released without the family knowing entirely. Yes, some of us had received it (legal letter sent to the family's solicitor), but the rest of us were at work. We didn't get that information straight away and to suddenly hear it being broadcast is disgraceful.
"And, again, it's just another thing that the government are trying to do to undermine us as a family and treat us with so much disrespect, which has happened right through the 28 years since my father was murdered."
Mr Brown's daughter said the only acceptable way forward for her family was a public inquiry.
"There's obviously very dirty secrets there that the government want to keep from us as a family and they're going out right, taking all action that they possibly can to prevent that information coming out," she said.
The Northern Ireland Secretary said the Court of Appeal judgment required a "comprehensive response" and he had asked for an extension to the grace period so he could provide it.
Mr Benn had resisted the call for a public inquiry and suggested that the Brown family engage with a new body established under the UK's controversial Legacy Act.
The Labour government promised to repeal and replace the act, which was drawn up under the previous Conservative government.
But it wants to retain the Independent Commission on Reconciliation and Information Recovery which will handle all Troubles cases.
Mr Benn had urged the Brown family to engage with the ICRIR.
But the Court of Appeal ruled that the commission had insufficient powers to properly investigate Mr Brown's murder and was not equipped to deal with the kind of sensitive material likely to be produced to it.
