An official decision to bring no charges against the policeman who struck Ian Tomlinson minutes before he died at the G20 protests is under intense scrutiny as it emerged that the Independent Police Complaints Commission had backed a prosecution for manslaughter.

Keir Starmer, the director of public prosecutions, acknowledged there was evidence that the officer, named as PC Simon Harwood, assaulted Tomlinson, 47, minutes before he died. But he said there was no realistic prospect of conviction because of "sharp disagreements" between pathologists.

The decision was met with fury by Tomlinson's family, who accused the authorities of a 16-month cover-up over the death of the seller on 1 April last year, when he was seen on video being struck by an officer and then shoved to the ground, despite behaving peacefully.

The Crown Prosecution Service's view clashes with that of an investigation by the Independent Police Complaints Commission. The Guardian has learned the IPCC concluded there was sufficient evidence to charge the officer with manslaughter, and told Tomlinson's family so.

The first postmortem by Dr Freddy Patel endorsed the police's version of events, ruling that he died from a heart attack.

But a direct challenge to the CPS also emerged last night from Dr Nat Cary, the second forensic pathologist who examined Tomlinson's body. He told the Guardian prosecutors made a factual error in dismissing a charge of actual bodily harm.

He said his report contained clear evidence that Tomlinson suffered injuries sufficient to support an ABHcharge. The CPS dismissed the injuries as "relatively minor" and thus not enough to support a charge of ABH in its written reasons given to the family.

Cary, speaking for the first time about the case, said: "I'm quite happy to challenge that. The injuries were not relatively minor. He sustained quite a large area of bruising. Such injuries are consistent with a baton strike, which could amount to ABH. It's extraordinary. If that's not ABH I would like to know what is."

The CPS said Patel's findings would provide a jury with enough reasonable doubt that the officer's strike contributed to the death, and as a result they would acquit. By coincidence Patel yesterday faced a disciplinary hearing at the General Medical Council for allegedly conducting four other autopsies incompetently. He could be struck off and the Home Office has suspended him from its approved list.

Starmer said the CPS could not bring a charge of common assault because it failed to do so within a legal time limit .

Tomlinson's family accused the authorities of a "big cover-up" and there were heated exchanges as they met with prosecutors after being told the news.

Tomlinson's stepson Paul King said: "It's outrageous. We feel like it was not a full investigation from the beginning. It's a big cover-up.

"He has just admitted on TV that a copper assaulted our dad. But he hasn't done anything. He's the man in charge … why hasn't he charged him.?

The Tomlinsons' solicitor, Jules Carey, said the decision was disgraceful and said an inquiry must examine if it was due to a "lack of will or incompetence".

The solicitor said Cary's view that the CPS made factual errors would be examined to see if it could form part of a legal challenge: "The family were surprised about how the extent of the injuries were minimised by the CPS."

The family's expectation that the officer would be charged was built on the video evidence and because of what the IPCC told them about its investigation.

The IPCC concluded its investigation into the death and handed its file to the CPS in August 2009. Shortly after, senior investigators held a meeting with the family to discuss their findings. While they made clear the CPS was responsible for charging decisions, IPCC officials told the family they believed there was sufficient evidence to charge the officer with manslaughter.

Last night the IPCC said: "The officer was interviewed for the offence of manslaughter under caution." An inquest will now be held into the death, where the family will hope a jury hear the case. The officer remains suspended and is expected to face a disciplinary hearing.

Deborah Coles of the Inquest charity said: "The eyes of the world will be looking on with incredulity as yet again a police officer is not facing any criminal charges after what is one of the most clear-cut and graphic examples of police violence that has led to death. This decision is a shameful indictment of the way police criminality is investigated."

The CPS lawyer who made the decision was the same one who decided no officer should face charges for the shooting dead of Jean Charles de Menezes by police who mistook him for a terrorist. That shooting happened five years ago yesterday.

The Met commissioner, Sir Paul Stephenson, said he regretted Tomlinson's death and offered his sympathy to his family. He said he was concerned by the video footage but that it was not appropriate for him to comment on the outcome of the IPCC inquiry or the CPS decision.