Dr Kenneth Shorrock, who carried out post-mortem examinations after Mr de Menezes was killed, wrongly recorded in his notes that the Brazilian jumped over a barrier before "stumbling" down an escalator in the moments before officers opened fire.
He said he was given the false information during a "walk-through" with officers at Stockwell Tube station, south London, in the hours after the shooting.
When asked why there were "significant errors" in his initial report, he replied: "This was what was told to me.
"What happened at that time was that there were a lot of officers present and we were taken through.
"I did not write anything down. I did not make any note of who told me what - but, at the next opportunity that I had, I got my Dictaphone."
He told a jury at the Oval cricket ground, south London, that "it was a long day" but rejected suggestions he was influenced by media reports, insisting he had made the notes before he had seen any news items.
Police marksmen shot Mr de Menezes seven times in the head at point-blank range on a train carriage on July 22 2005.
He had been mistaken for one of the terrorists behind the previous day's failed suicide attacks on the capital.
Dr Shorrock was speaking before retired Deputy Assistant Commissioner Brian Paddick said the operation's designated leader, Cressida Dick, should only have been concerned with ordering a critical shot at Mr de Menezes.
The inquest has heard how Ms Dick, now Mr Paddick's successor as Deputy Assistant Commissioner, made a series of orders as surveillance officers followed the innocent Brazilian through south London.
But Mr Paddick, who unsuccessfully ran for election as the Liberal Democrat candidate for London Mayor in May, said the officer in charge of the control room should only deal with the decision to open fire.
He said: "My understanding of the role of the DSO (designated senior officer) is to make decisions around when officers are confronted by a suicide bomber who is suspected of being about to detonate a device.
"That officer is designated only to deal with the critical shot decision."
The retired police chief also said, under Scotland Yard policy, officers should warn suspects before opening fire if they were not convinced that they were a terrorist.
He added: "If, having been authorised to take the critical shot, the firearms officers have some doubt, there's an extra bit of information that clearly the DSO has not had, who's not there on the scene, then the firearms officers should give a warning and then respond to the suspect on the basis of how the suspect responds to that warning."
Dr Shorrock told the inquest Mr De Menezes would not have survived any of four bullet wounds to his brain.
He later told how he was not sure who told him Mr de Menezes had "vaulted" the barrier before he was shot.
He said he "cannot recall" if other interested parties were at the station during the walk-through.
Michael Mansfield QC, for the de Menezes family, asked him whether all information had to come from the senior investigating officer.
Dr Shorrock added: "I cannot recall whether it all came down from him or whether it came from others but was agreed by him or what it was."
Yesterday a jury heard how the train driver fled into a tunnel as police shot dead the innocent Brazilian because he believed firearms officers were terrorist "fanatics".
Mr Paddick told the inquest Ms Dick's order to "arrest him, but whatever you do, don't let him get on the Tube" was ambiguous.
Michael Mansfield QC, representing the De Menezes family, said firearms officers interpreted it as "he must be stopped before he gets on the Tube."
The retired senior officer said: "I think in the scenario that we had, where you have got a DSO, you have got officers with unusual ammunition, you have got a suspected suicide bomber, that order is ambiguous and it could mean one or two things depending on your frame of mind as to how you interpret it."
He also said that Commander Jim Carter showed him clear surveillance photos of failed bomber Hussain Osman on July 23.
These were much better quality than the CCTV stills released to the press the previous day, he said.
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