London’s Metropolitan Police has been accused of “institutional corruption” by an inquiry into one of the UK’s most notorious unsolved murders, involving cover-ups, private investigators and the media.

The report released on Tuesday into the murder of private detective Daniel Morgan in 1987 by an independent panel found the Met’s priority in its investigations had been to safeguard its reputation, rather than to reveal how he came to be killed or to provide closure to his family, who have campaigned for decades for the truth.

“We believe that concealing or denying failings for the sake of an organisation’s public image is dishonesty on the part of the organisation for reputational benefit and constitutes a form of institutional corruption,” said Nuala O’Loan, the panel’s chair, in a statement.

The Met’s commissioner, Dame Cressida Dick, said it was a matter of “great regret” no one had been brought to justice and that the force’s mistakes had compounded the pain suffered by Morgan’s family.

“For that I apologise again now,” she said.

The report was also highly critical of the four separate police investigations and the inquest into the murder, all of which were the subject of serious criticism in the 1,200-page report.

The Met accepted corruption had tainted the first investigation, in which officers did not even conduct a proper search of the murder scene. “We accept corruption and the malicious acts of corrupt individuals were a major factor in the failure of the first investigation,” the force said in a statement.

Morgan was found dead on March 10 1987 with an axe embedded in his neck in the car park of the Golden Lion pub in Sydenham, south-east London. The case immediately raised suspicions of potential involvement by police officers who had been closely associated with Southern Investigations, his business.

O’Loan said the findings were relevant to the modern force, which had not shown “candour” about previous investigations into the killing.

She said she regarded the conclusion as of similar significance to those of the 1999 MacPherson Report into the failings of the Stephen Lawrence murder inquiry, which branded the force “institutionally racist”.

In reply to questions, O’Loan was highly critical of the force’s continued failure to vet serving officers and said there could be no assurance that similar corrupt practices to those that dogged the previous investigations were not still taking place.

“Although we have seen evidence of almost a cycle over the years of anti-corruption measures, we have not seen evidence such that we are satisfied that what happened in the past 34 years might not continue to happen,” she said.

“We just have not seen any evidence that although there are policies there, they actually seem to be effective in stopping things happening,” she added.

Home secretary Priti Patel told the House of Commons on Tuesday that the episode had been “one of the most devastating” in the force’s history. She said she had asked Dick to provide her with a “detailed response”, and was commissioning a report from HM Inspectorate of Constabulary and Fire and Rescue Services on police corruption.

In 2013, Theresa May, then home secretary, ordered a formal inquiry in response to calls from Alastair Morgan, Daniel’s brother, following the collapse of a trial over the murder.

The defendants at the trial, which was stopped when much of the evidence was ruled inadmissible, included Jonathan Rees, Daniel Morgan’s business partner, and two of Rees’s brothers-in-law Garry and Glenn Vian. A former Metropolitan Police officer, Sidney Fillery, was charged with perverting the course of justice at the trial, though this was later dropped.

The report said it could not rule out suggestions that the murder had been committed where it was to ensure it would be investigated by detectives based at Catford, where Fillery was based and whose detectives were regular drinking partners of Rees.

While the report focused mainly on the police, there was also substantial material on links between Southern Investigations and the News of the World, the now-defunct Sunday newspaper. The company received significant payments from the paper for information, much of it gleaned from police officers.

The report highlighted how the newspaper came to pay Southern Investigations to conduct surveillance on David Cook, the officer in charge of the second investigation into the murder.

It said circumstantial evidence suggested “very strongly” the surveillance had been arranged by Fillery, then working at Southern Investigations, and Alex Marunchak, a former senior News of the World executive, with a view to discrediting or intimidating Cook.

O’Loan was critical of the Met’s co-operation with the inquiry, citing its reluctance to allow unfettered access to the Holmes serious investigation database. She said that some information had not been disclosed until March this year.

Dick made some of the initial decisions about access to Holmes while assistant commissioner of the Met. She insisted on Tuesday that she had been “personally determined” that her force should provide the inquiry with “the fullest level of co-operation in an open and transparent manner, with complete integrity at all times”.

She also said it was still possible there would be a conviction over the murder.

However, Alastair Morgan said at a press conference after publication that Dick should resign.