The Independent Office for Police Conduct (IOPC) said: "She appears to have acted in the genuine belief she had a legitimate policing purpose, due to concerns about protecting the information, but may have got it wrong by prioritising those concerns over her duty to facilitate full and exceptional disclosure."

But the IOPC said there was no evidence Dame Cressida was trying to protect corrupt officers, the information was eventually handed over, and her behaviour would not have merited disciplinary action if she was still serving.

Dame Cressida resigned as Met Police chief in February after a row with London mayor Sadiq Khan, but was an assistant commissioner at the time that the Daniel Morgan Inquiry Panel asked for the information.

Mr Morgan was a private investigator when he was found dead with an axe in his head in the car park of the Golden Lion pub near his office in Sydenham, south London, in 1987.

Nobody has been successfully prosecuted for the murder after four major homicide investigations, an inquest, and several disciplinary inquiries.

The independent panel reported last year, eight years after it was set up to probe the Met's handling of the case, and damned the force as "institutionally corrupt" and accused Dame Cressida of holding up its inquiry.

The IOPC then assessed the panel report to see if there was still any scope for criminal or disciplinary charges, but found none after considering evidence involving 50 serving or retired officers.

The watchdog did criticise former assistant commissioner John Yates, who retired in 2011, for failing to take action against the senior detective whose own actions led to the exclusion of key witness evidence in the second of the four murder investigations.

Sal Naseem, IOPC regional director for London, said: "From the first to last investigation into Daniel Morgan's murder, there were failures to adequately challenge and investigate allegations that officers had acted corruptly.

"In coming to our decisions, we are acutely aware that not one single officer was ever successfully prosecuted or received significant disciplinary action as a result of corruption directly connected to the murder investigations.

"The wrongs that occurred can never be put right, but it may have served as some small comfort to Mr Morgan's family and loved ones if the officers involved had been held to account and suffered the consequences of their actions at the time.

"The circumstances of these matters must serve as a salient reminder to the Metropolitan Police and the police service more widely, of the importance of being constantly vigilant in challenging improper and corrupt behaviour swiftly, firmly and robustly."

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Dame Cressida said: "This conclusion concurs with the assessment made by the Mayor's Office for Policing and Crime many months ago. The IOPC recognise that everything I did was for a legitimate purpose.

"They also recognise that in providing 'full and exceptional disclosure' to the panel I had to fulfil other legal responsibilities - most importantly, not to disclose inappropriately information that would put lives at risk.

"I disagree with their analysis that my actions 'may give an indication of a breach of standards of professional behaviour' and that 'I may have got the balance wrong'.

"In the period from September 2014 to January 2015, the record shows I and my team acted professionally, flexibly, expeditiously, diligently and with integrity in a challenging, unprecedented and complex task. By December 2014, the Panel had received 133,000 pages of material.

"Finally, and most importantly, I deeply regret that no one has been brought to justice for Daniel's murder and regret everything the Met or any of its members have done which has added to the pain of Daniel's family of losing Daniel in such terrible circumstances."

The Metropolitan Police again apologised for its failure to bring Mr Morgan's killers to justice, and said it recognised the part played by corrupt officers in undermining public confidence in the force. It said it had since reformed investigations into homicide, misconduct, and corruption.

Assistant commissioner Amanda Pearson said: "We are always open to learning and are studying this report to understand how it can shape our work."