The Independent Police Complaints Commission is asking the High Court to quash its own report clearing police officers of any serious wrongdoing following the death of a man shortly after he was Tasered.
Factory worker Jordon Begley, 23, from Gorton, Manchester, died in July 2013 two hours after being shot at his home with a 50,000 volt stun gun from a distance of 28in (70cm). He was also punched and restrained by armed officers, who believed he had a knife.
In the first case of its kind, lawyers for the IPCC told two judges sitting in London the report on the investigation into Mr Begley's death was legally flawed and there should be "a new and lawful" investigation.
The 2014 report under challenge concluded no officers should be prosecuted or face misconduct proceedings. The officers involved are opposing the IPCC request to have the report overturned.
Jeremy Johnson QC, appearing for both the IPCC and its chief executive, said there had been "a serious departure" from statutory requirements, and the investigator who prepared the report had not applied the correct legal test and did not accurately summarise the evidence, or attach or refer to all relevant documents from the investigation.
The QC told Lord Justice Elias and Mr Justice Males a new investigation is the appropriate way of vindicating the right of Dorothy Begley, the mother of the dead man, to a proper inquiry under Article 2 of the European Convention on Human Rights.
An inquest jury delivered a narrative verdict in July 2015, saying Mr Begley had been "inappropriately and unreasonably" Tasered and restrained. The jury found officers were "more concerned with their own welfare" than Mr Begley's.
Ms Begley, who attended the High Court hearing on Wednesday, said: "It has been more than three years and I am still fighting for someone to say sorry for destroying my family and taking my son's life.
"I will fight on not matter however long it takes. I will never give up."
Mr Johnson said the 2014 report had inadequately dealt with evidence on whether the five officers involved should face misconduct proceedings.
The report did not summarise contradictory statements from the officers on the position of Mr Begley's hands before he was Tasered.
The issue was potentially important to the question of whether the officers' use of force was justified because of fears that Mr Begley had a knife when they were called to his home, said Mr Johnson.
One officer had said Mr Begley's hands were in his pockets while another said they were clenched in fists.
The report also did not summarise evidence that one of the officers had said he delivered two "distraction strikes" to Mr Begley and that this "knocked the wind" out of him while efforts were continuing to subdue him.
This also went to the question of whether excessive force was used.
Mr Johnson argued these failures in the report meant Greater Manchester Police and the IPCC commissioner did not have a sufficient summary of the evidence to make the decisions they were required to make.