The brother of murdered private investigator Daniel Morgan has said his mother "never stopped fighting" to seek justice for her son but died without ever seeing anyone convicted for his death.
Mr Morgan was found outside a London pub car park with an axe in his skull in March 1987 and his death remains unsolved.
Isobel Hulsmann campaigned to uncover the truth about her son's murder, and often travelled from her home in Hay-on-Wye, Powys, to lay flowers at her son's grave in London.
An independent panel was set up in May 2013 after numerous inquiries, at an estimated cost of £30 million, had failed to shed light on Mr Morgan's murder.
Then-home secretary Theresa May appointed former Northern Ireland police ombudsman Baroness Nuala O'Loan of Kirkinriola to lead the panel.
Alastair Morgan, the brother of Mr Morgan, revealed the death of his mother on Twitter on Friday, saying: "My dear mother and fellow justice campaigner Isobel Hulsmann passed away peacefully yesterday. RIP Mum xxxx"
He said that his mother, 89, was diagnosed with cancer six months ago, adding: "She had a long life, but it's caused us all great stress and distress throughout, but my mother never stopped fighting.
"Until fairly recently she was attending all the meetings, but in the last six months she was too ill."
Scotland Yard admitted that the first inquiry into the 1987 killing had been hampered by police corruption.
Five people were arrested in 2008 but two, including a former detective accused of perverting justice, were discharged after a string of supergrasses were discredited.
In a written ministerial statement in July 2014, Mrs May called the case "one of the country's most notorious unsolved murder cases".
At the time, she added: "After numerous separate police investigations into the case between 1987 and 2002, the Crown Prosecution Service discontinued the attempted prosecution against five suspects in 2011.
"The Metropolitan Police admitted that police corruption was a 'debilitating factor' in the original investigation."
The panel set up in 2013 aims to shine a light on the circumstances of Mr Morgan's murder, its background and the handling of the case over the period since 1987. Alastair Morgan believes the report will be published next autumn.
A spokesman for the Metropolitan Police said the force continues to provide as full as possible support to the panel.
See full story in the County Times