A report by Metropolitan Police Commissioner Sir Ian Blair into the handling of a Welsh private detective's murder has been branded inadequate.
Daniel Morgan's brother said he was surprised by the MPA's decision
Commissioned by the Metropolitan Police Authority (MPA), it examined the 1987 killing of Daniel Morgan, 37, found with an axe in his head in a south London car park.
MPA chairman Len Duvall said it failed to answer "the key issues of concern".
The Met force said it would try to address his concerns.
Daniel Morgan's brother, Alastair, from London, said he was surprised by the MPA's decision.
The body of Daniel Morgan, who was originally from the village of Llanfrechfa, Monmouthshire, was found near the Golden Lion pub in Sydenham.
The family - whose members include his mother, Isobel and sister, Jane, of Hay-on-Wye, Powys - all believe Mr Morgan was killed by a professional hitman after uncovering evidence of police corruption.
But the family's allegation that he had found corruption was never substantiated by the former Police Complaints Authority and a series of investigations failed to lead to any prosecutions.
But following years of pressure from the Morgan family, Met commissioner Sir Ian was asked to produce a report on the case last October.
As a result of the MPA's ruling about that report, it is now possible that a barrister will now be appointed "to independently review all the case papers in relation to the murder and all subsequent investigations".
Informing Alastair Morgan about the latest development, the MPA said: "It is his (Mr Duvall's) view that the report is not adequate, for example in either reaching an understanding of past investigations or in acknowledging how possible misconduct by one or more officers may have affected the investigation of this murder.
"Nor does he think it answers the key issues of concern for you and your family.
"The MPS (Metropolitan Police Service) will therefore be informed tomorrow (Friday) that he is not prepared to accept the report as it stands.
Isobel Hulsmann, Daniel Morgan's mother, at the spot where he died
"Len fully appreciates quite how upsetting this will be for you and your family but wants you to know that he remains fully committed to taking this forward to a positive conclusion."
The MPA added that Mr Duvall would meet the Morgan family next Friday where he would discuss "possible ways forward".
An MPS spokeswoman said: "We have been made aware of the concerns of the chair of the MPA and will seek to address these in due course."
Alastair Morgan said he welcomed the MPA's decision.
"I did not expect it and I was surprised to be honest," he said.
"I am pleased they are showing more backbone than the Home Office has shown on this issue. Len Duvall has acted commendably throughout."
Daniel Morgan was the co-owner of a firm called Southern Investigations, which employed off-duty police officers.
During a series of police investigations since his death, a business partner of Mr Morgan was arrested and charged but the charge was dropped.
Several serving police officers were also arrested but at Mr Morgan's inquest they were exonerated by the coroner and two were later paid damages for false imprisonment.
There was a further series of arrests in London and Croydon in late 2002, followed by the arrest of a former Metropolitan Police officer in his 50s in January 2003 on suspicion of "misconduct in a public office" relating to the initial murder inquiry.
But in September 2003, the Crown Prosecution Service concluded there was insufficient evidence to mount a prosecution.