The pathologist who claimed Ian Tomlinson died of a heart attack is being investigated by the medical watchdog after an inquest jury disputed his findings.
An inquiry has been launched by the General Medical Council into how Dr Freddy Patel's post-mortem examination differed with two other experts who said he died of internal bleeding.
Dr Patel was accused at Mr Tomlinson's inquest of making blunders which made it all but impossible to conclude with any certainty how he died on the fringes of the G20 protests in London on April 1 2009.
Dr Patel's claim that Mr Tomlinson died of a heart attack was dismissed by a string of experts who said he died of internal bleeding.
There was no formal announcement from the GMC but a source confirmed investigations into Dr Patel's conduct were under way.
An inquest jury concluded Mr Tomlinson died of the bleed as a result of Pc Simon Harwood's "excessive and unreasonable" force in shoving him to the ground.
Dr Patel's notes are alleged to be ambiguous and he did not order tests on three litres of fluid found in Mr Tomlinson's abdomen to confirm whether it was pure blood or - as he maintained - largely made up of a substance called ascites produced by liver disease.
This question was crucial because fellow pathologists Dr Nat Cary and Dr Kenneth Shorrock disputed Dr Patel's findings that the cause of death was coronary artery disease, consistent with natural causes.
An inquest jury which delivered an unlawful killing verdict earlier this week agreed with the other experts' conclusions.
News of the investigation comes after Britain's most senior police officer denied he was jumping to judgment and using Pc Harwood as a "fall guy".
Sir Paul Stephenson was accused of being prepared to "surrender" the officer by the Ulster Unionist peer Lord Maginnis, who added he was "horrified" by the police stance.
The Director of Public Prosecutions Keir Starmer QC is to examine whether there is enough evidence to charge the Scotland Yard officer with manslaughter.
Sir Paul said: "It has to be a matter of very real regret for any commissioner to be in a situation where there has been a finding that somebody has died as a result of one of his officers.
"That's a simple statement. It's fact. That's not jumping to a judgment."
But it was right to recognise the "observations and conclusions" of the jury, the commissioner said.
Lord Maginnis said: "It's worried me that you, as a leader of the police, were prepared to sacrifice a constable who I believe should have the full support of the police service," he told Sir Paul.
Jurors delivered a damning assessment of the Scotland Yard officer's actions, saying he "deliberately and intentionally" shoved the newspaper seller to the ground.
Pc Harwood, a father-of-two, will now become the first officer to face a public Metropolitan Police disciplinary inquiry.
In a written statement the jurors told the inquest: "Both the baton strike and the push were excessive and unreasonable.
"As a result, Mr Tomlinson suffered internal bleeding which led to his collapse within a few minutes and his subsequent death.
"At the time of the strike and the push, Mr Tomlinson was walking away from the police line. He was complying with police instructions to leave Royal Exchange Buildings, the passage. He posed no threat."