A police officer seen striking Ian Tomlinson during G20 protests could face manslaughter charges after it was revealed tonight the newspaper vendor died of internal bleeding.
A second post-mortem examination, carried out at the request of the 47-year-old's family, found he suffered an "internal haemorrhage" and not a heart attack as previously thought.
The police constable captured on video footage hitting Mr Tomlinson with his baton before pushing him to the ground is suspended from duty and could now be prosecuted.
A spokesman for the Independent Police Complaints Commission (IPCC) said: "Following the initial results of the second post-mortem, a Metropolitan Police officer has been interviewed under caution for the offence of manslaughter as part of an ongoing inquiry into the death of Ian Tomlinson."
Mr Tomlinson's son Paul King accused the officer, a member of the Territorial Support Group, of violently assaulting his father.
He said: "First we were told that there had been no contact with the police, then we were told that he died of a heart attack.
"Now we know that he was violently assaulted by a police officer and died from internal bleeding.
"As time goes on, we hope that the full truth about how Ian died will be made known."
The family's solicitor, Jules Carey, called for the case to be sent to prosecutors promptly for them to consider charges.
The first post-mortem, which was carried out by pathologist Dr Freddy Patel found Mr Tomlinson, a recovering alcoholic, had a diseased liver and heart. He identified the cause of death as a heart attack.
But Dr Nat Cary, who carried out the second post-mortem, rejected that conclusion in his provisional findings, City of London Coroner's Court said.
He said the hardening of the arteries in Mr Tomlinson's heart was not serious enough to kill him.
A spokesman for the coroner said: "Dr Cary's opinion is that the cause of death was abdominal haemorrhage. The cause of the haemorrhage remains to be ascertained.
"Dr Cary accepts that there is evidence of coronary atherosclerosis but states that in his opinion its nature and extent is unlikely to have contributed to the cause of death."
A medical expert said Mr Tomlinson's treatment by police could easily have led to the internal bleeding that appears to have killed him.
Dr Charles O'Donnell, consultant in emergency and intensive care at Whipps Cross University Hospital in London, said an existing condition could have left Mr Tomlinson vulnerable to the effects of an attack.
Scotland Yard said the force was co-operating fully with the IPCC and would "proactively" give it any relevant information.
He said: "The Metropolitan Police Service wishes to reiterate its sincere regret in relation to the death of Ian Tomlinson.
"Our thoughts are with his family, and all those affected by this tragedy.
"As an independent investigation by the Independent Police Complaints Commission into the circumstances surrounding police contact with Mr Tomlinson is ongoing, we are unable to comment specifically on the findings of the post-mortem.
"We continue to cooperate fully with the IPCC and proactively provide any information that may assist them. We await the findings of the investigation."
In the aftermath of Mr Tomlinson's collapse, Scotland Yard issued a statement describing how officers trying to help him were pelted with bottles by protesters.
But under pressure following the revelations surrounding his death, and footage of another protester being struck by an officer, Met Commissioner Sir Paul Stephenson ordered a review of police tactics in handling demonstrators.
Mr Tomlinson is believed to have come in to contact with police when he was stopped from trying to leave the area in front of the Bank of England that were the focus of the protests.
The results of the post-mortem have been known for a week but were not made public because of IPCC fears it would prejudice the investigation, Mr Carey said.
"The video footage of the unprovoked and vicious assault on Ian by the police officer would easily justify charges of assault being brought against the officer," he said.
"The findings of Dr Cary significantly increase the likelihood that the officer will now face the more serious charge of manslaughter.
"The family have been aware of the findings of the second pathology report for a week and have had to endure the holding back of this information despite continuing reports in the press that Ian died of a heart attack.
He said the IPCC at first opposed revealing the findings, fearing it might prejudice their investigation.
"It is of some comfort to the family that the record is now being put straight, but they hope that the IPCC investigation will be expedited and thorough, and that there will be a prompt referral to the CPS for charge," he added.
London mayor Boris Johnson said: "There must now be a fast and transparent conclusion to the IPCC investigation, with the full and urgent co-operation of all involved."