• G20 pathologist reprimanded over conduct • MP asks why first post mortem was held so quickly, and calls for public inquiry

The initial post mortem examination of the man who died at the G20 protests after being attacked by a police officer, which found he had died of a heart attack, was conducted by a forensic pathologist once reprimanded about his professional conduct by the General Medical Council.

Ian Tomlinson, a 47-year-old newspaper seller, died on April 1 after being assaulted at least once by officers policing the G20 demonstrations. He had been trying to walk home from work when he was confronted by police, hit with a baton and thrown to the ground.

Two days later Home Office pathologist Dr Freddy Patel concluded Tomlinson had died of a heart attack. He has previously been reprimanded by the GMC, after he released medical details about a man who died controversially in police custody.

In a second case, which raised questions about Dr Patel's findings, police dropped a criminal investigation after the pathologist gave it as his opinion that the victim, a woman, had died of natural causes. A man who lived in the flat where the body was found went on to murder two other women and mutilate their bodies.

The Independent Police Complaints Commission (IPCC) launched a criminal inquiry into Tomlinson's death after footage obtained by the Guardian showed he had been attacked from behind. Tomlinson had had his hands in his pockets and was walking away from police lines.

Last night a second post mortem examination, intitiated by Tomlinson's family and the IPCC, was being carried out by Dr Nat Cary.

A source with detailed knowledge of the IPCC investigation expressed surprise that the initial post mortem was referred to Dr Patel rather than the Forensic Pathology Services, a body of nine independent forensic pathologists, including Dr Cary, which usually deals with suspicious deaths in London and the home counties.

The selection of Dr Patel could have been a jurisdictional issue relating to City of London authorities, the source said.

"Who chose the first pathologist and how?" said David Howarth MP, the Liberal Democrats' justice spokesman. "When the first post mortem came out saying natural causes so soon after his death, I thought that was too quick." He added: "At the very least, there now needs to be a full and independent public inquiry into the Tomlinson case, and the actions of the public authorities in the days after his death. Who knew what - and when?"

The IPCC took over the criminal investigation into the death of Ian Tomlinson on Wednesday, after the Guardian published footage showing he had been hit with a baton and thrown to the ground by an officer in an apparently unprovoked assault. The Guardian included the video in a dossier of evidence handed to the IPCC that also contained statements from witnesses who alleged Tomlinson was assaulted by police at least once before he died.

Dr Patel is on a Home Office register of accredited forensic pathologists, which is managed on behalf of all police forces by the National Policing Improvement Agency. Questions have twice been asked about his handling of suspicious death cases. In 1999 Dr Patel was disciplined by the GMC after he discussed the medical history of Roger Sylvester, a 30-year-old black man who died in police custody, outside an inquest hearing.

He told reporters: "I am aware from the medical records held at Whittington hospital that Mr Sylvester was a user of crack cocaine." Sylvester's family were devastated by the suggestion and contested that he been a user.

Asked about the GMC reprimand last night by telephone, Dr Patel said: "That is right. It happened a long time ago."

In a separate case in January 2002, Dr Patel found that Sally White, 38, died of a natural causes resulting from heart disease. Her body was found locked in a bedroom in the Camden flat of Anthony Hardy, a 52-year-old psychiatrically-disturbed alcoholic.

White's death was treated as suspicious until Dr Patel detailed the cause of her death as a heart attack. Hardy went on to kill two women, Elizabeth Valad, 29, and Brigette MacClennan, 34, and place their body parts in bin bags.

Last night, Dr Patel said: "As far as I know, my findings [in the Sally White case] stand as they were, and I wasn't criticised." He declined to comment on whether he was the correct person to conduct the first post mortem examination into Tomlinson's death, referring questions to the coroner. The City of London coroner in the case, Paul Matthews, was yesterday not contactable.

Last night a spokesperson for the City of London said they did not have the necessary information to comment on Dr Patel's involvement in Tomlinson's post mortem. An IPCC spokesperson said: "Again, we are not willing to give a running commentary of an ongoing criminal investigation as we wish to maintain the integrity of the investigation."