Margaret McQuillan, sister of Ms Smyth-Campbell, initiated judicial review proceedings against the PSNI in 2015 after documents uncovered at the National Archives linked the murder to an undercover British military unit.

On Friday, Mr Justice Paul Maguire found that past investigations - both the original RUC investigation and a 2008 investigation by the Historical Enquiries Team - were fatally flawed.

The court also agreed that the PSNI’s Legacy Investigation Branch (LIB) could not be independent for the purposes of carrying out a proper investigation in line with article 2 of the ECHR.

Mr Justice Maguire said that “the proposed investigation by the LIB of Mrs. Smyth’s death conflicts with the requirements of Article 2 ECHR as the LIB lacks the requisite independence required to perform an Article 2 complaint investigation in respect of this death”.

Mrs McQuillan’s lawyer, Niall Ó’Murchú of Madden & Finucane Solicitors, said: “This is a very important judgment for not only the Campbell family but also potentially many other families who are seeking properly independent and article 2 compliant investigations into their loved ones’ deaths.

“The Campbell family are very relieved that this nearly three-year battle is over. They are relieved that an investigation of Jean’s death, which does not involve the PSNI may now proceed, and they feel they finally have obtained some justice for Jean.”