Murdered solicitor Pat Finucane's widow is to seek to overturn a ruling that David Cameron acted lawfully in refusing to hold a public inquiry into the killing.
Geraldine Finucane has lodged an appeal against the High Court verdict that the British government was justified in reneging on a previous commitment.
Her lawyer, Peter Madden, claimed internal communications shows former Secretary of State Owen Paterson had "closed his mind" to an inquiry.
Mr Finucane was gunned down by loyalist paramilitaries in front of his wife and their three children at their north Belfast home in 1989.
His family have campaigned for a full examination of alleged security force collusion.
Mrs Finucane took the Prime Minister to court after he ruled out a public inquiry in 2011.
Instead, Mr Cameron commissioned QC Sir Desmond de Silva to review all documents relating to the case.
Sir Desmond's report confirmed agents of the state were involved in the murder and that it should have been prevented.
However, it concluded there had been no overarching state conspiracy.
The Finucane family rejected the findings as a whitewash and accused the government of reneging on previous Labour government commitments.
Earlier this year Mr Justice Stephens ruled that Mr Cameron acted lawfully.
He found that Mrs Finucane had received a clear promise that an inquiry would take place but backed the government's case that other public interest issues, including political developments in Northern Ireland and the potential financial pressures of a costly inquiry, were enough to frustrate her expectation.
However, the judge also said the state has not fully met its human rights obligation to investigate.
Now the case is to go before the Court of Appeal for a further hearing.
Mr Madden said: "A full independent and international tribunal of inquiry, where documents will examined in public and witnesses shall be compelled to attend and be cross-examined by Geraldine's lawyers, remains the only model capable of achieving the truth of Pat's murder."
