The pathologist at the centre of a row over the death of a man at the G20 protests made mistakes in other post-mortem examinations, an inquiry says.
The General Medical Council found Dr Freddy Patel had been "irresponsible" in earlier, unrelated examinations.
He could be suspended or struck off the medical register at a later hearing.
Dr Patel's findings on the death of Ian Tomlinson differed from those of two colleagues, and led to no charges being brought over the death at the G20 demo.
Mr Tomlinson's family said they were not surprised by the GMC ruling, which raised "serious questions".
In a statement they said: "This raises the most serious questions about how and why Freddy Patel's appointment to carry out the vital first post-mortem on Ian could have been approved by the City of London Police, who paid part of his fee.
"No family can rest with this kind of stain hanging over the investigation of their loved one and we demand answers."
But the City of London Police said officers were unaware of any questions over Dr Patel's professional ability in the spring of 2009.
The GMC inquiry looked into four post-mortem examinations conducted by Dr Patel between January 2002 and December 2005.
It found he had been "liable to bring the profession into disrepute in regard to some of the accusations against him".
Dr Patel, 63, was said by the disciplinary panel to have failed to identify marks on the body of a five-year-old girl which suggested she had been violently attacked prior to her death.
The panel's chairman, Richard Davies, said Dr Patel's report into the death of the girl, who was admitted to hospital with a head injury following what was said to be a "serious fall", gave no details of so-called "marks of violence".
Mr Davies said: "If there were no significant marks of violence in your view, by implication there were some marks of violence."
He said the panel considered it "probable" that Dr Patel "performed only a cursory external examination of the body".
Dr Patel was also said to have behaved in a way which was liable to bring the profession into disrepute when he changed the cause of a woman's death to satisfy her family.
After carrying out a post-mortem examination on 5 January, 2005, he decided she died from a blood clot in the coronary arteries.
But a month later, after a second post-mortem examination by another pathologist, he changed the cause of death to a brain haemorrhage.
The GMC panel said it was not persuaded that it had been "professionally scrupulous" to make the alteration.
Mr Davies said Dr Patel had told an inquest into the woman's death that he had made the change to "satisfy the family". The panel concluded he had behaved irresponsibly.
Dr Patel was the pathologist who carried out the first post-mortem examination of newspaper seller Ian Tomlinson, who died at the G20 protests in London on 1 April 2009.
He concluded that Mr Tomlinson died of natural causes linked to coronary artery disease.
However, two other pathologists later separately concluded that Mr Tomlinson - who had been pushed by a police officer - died of internal bleeding as a result of blunt force trauma, in combination with cirrhosis of the liver.
The failure of the three pathologists to all agree on the cause of Mr Tomlinson's death led to the director of public prosecutions ruling that the policeman in question would not face any charges.
Dr Patel has since been barred from carrying out Home Office forensic pathology work.
A City of London Police spokesperson said: "Pathologists are appointed by a coroner and police forces have the right to overrule the coroner's choice of pathologist when requesting forensic post-mortems.
"Officers were not aware of any questions over Dr Patel's professional ability in the spring of 2009, and therefore had no reason to question the coroner's decision.
"Following a review of all the force's forensic support in the summer of 2009, an agreement was reached with a forensic pathology service-provider to supply pathologists."