A police officer who was filmed pushing a man to the ground during the G20 protests will not face charges over his death.
Ian Tomlinson, 47, died after being caught up in the clashes on 1 April 2009 in the City of London.
Director of Public Prosecutions Keir Starmer said there was no prospect of conviction because experts could not agree on how Mr Tomlinson died.
Mr Tomlinson's son Paul King described the decision as "outrageous".
The officer who was filmed pushing Mr Tomlinson has been named as Pc Simon Harwood from the Metropolitan Police territorial support group.
Mr Starmer said there was a "sharp disagreement between the medical experts" about the cause of death, which led to three post-mortem examinations being conducted on Mr Tomlinson.
The first examination by Dr Freddy Patel - currently under investigation for alleged misconduct over four unrelated post-mortem examinations - found he died of natural causes linked to coronary artery disease.
The second pathologist, Dr Nat Cary, found he died of internal bleeding as a result of blunt force trauma, in combination with cirrhosis of the liver.
The third examination agreed with the findings of the second test. It was conducted on behalf of the officer.
Mr Starmer said there were irreconcilable differences between the evidence from Dr Patel and the two subsequent post-mortem examinations.
Dr Patel already faces disciplinary proceedings, and could be struck off, by the General Medical Council over alleged failings in his handling of four separate post-mortem examinations between 2002 and 2005.
Mr Tomlinson, a newspaper seller who was not involved in the protests, was walking home when he was caught up in the demonstration.
The video footage showed him being apparently struck by a baton and then pushed to the ground.
He was seen moving away after the incident but was found collapsed 100 metres away in Cornhill.
Mr Starmer also said that Mr Tomlinson was bitten by a police dog shortly before the clash.
Setting out the details of the decision, Mr Starmer said: "After a thorough and careful review of the evidence, the CPS (the Crown Prosecution Service) has decided that there is no realistic prospect of a conviction against the police officer in question for any offence arising from the matter investigated and that no charges should be brought against him.
"In the face of this fundamental disagreement between the experts about the cause of Mr Tomlinson's death, the CPS embarked on a detailed and careful examination of all the medical evidence and held a series of meetings with experts in attempt to resolve, or at least narrow, the areas of disagreement.
"This inevitably took some considerable time," he added.
He added the CPS had considered assault charges but prosecutors felt that they could not prove the push substantially harmed the newspaper vendor.
A charge of common assault, which does not require proof of injury, could not be brought against the officer because there is a six-month time limit.
Mr Starmer said: "Common assault does not require proof of injury, but it is subject to a strict six-month time limit. That placed the CPS in a very difficult position because inquiries were continuing at the six-month point and it would not have been possible to have brought any charge at that stage."
The CPS also decided not to charge the officer, who remains suspended from duty, with misconduct in a public office.
Mr Tomlinson's son Mr King said: "It's taken 16 months to get a no-charge against this officer.
"The CPS are clearly admitting the police officer assaulted our dad.
"We feel like it wasn't a full investigation from the beginning. It's been a big cover-up and they're incompetent.
"Why isn't there an assault charge? We feel very let down, very disappointed.
"We expected a charge. It clearly shows our dad being assaulted by a police officer," he added.
Mr Tomlinson's family solicitor Jules Carey said the family will consider whether they can appeal against the decision.
He said: "The CPS have accepted the conduct of the officer was unlawful.
"We now need to find out if there has been a lack of will or incompetence, and frankly there needs to be an inquiry into that."
Jenny Jones, a member of the Metropolitan Police Authority, said: "It's clearly an outcome that satisfies absolutely nobody and everybody comes out of it badly.
"The reputation of the police is poor, and morale won't be very good if public perception is that the police constantly get away with crimes and are never brought to justice.
"If everybody had moved a bit faster we might have actually been in the time-frame for an assault charge to be brought," she added.
Expressing "regret" for Mr Tomlinson's family, a Metropolitan Police spokesman, said: "There will, of course, be an inquest where the facts will be heard publicly. This is important for the family of Ian Tomlinson as well as Met officers and Londoners.
"We now await the IPCC's investigation report before being able to carefully consider appropriate misconduct proceedings," he said.
Deborah Glass, from the Independent Police Complaints Commission, said the circumstances of Mr Tomlinson's death will now be "rightly scrutinised" at an inquest.
She said: "We will provide a report on the officer's conduct to the Metropolitan Police within the next few days.
"The Met will need to provide us with its proposals regarding misconduct."