The police officer caught on film attacking Ian Tomlinson during the G20 protests could face manslaughter charges after a second postmortem concluded that the newspaper vendor died from internal bleeding and not a heart attack.

The dramatic new evidence, made public yesterday, provoked an immediate response from the victim's family, who said that they had been "badly misled" by police.

It emerged last night that the Metropolitan police officer who had been suspended from duty has now been interviewed under caution on suspicion of manslaughter by investigators from the Independent Police Complaints Commission.

Yesterday's developments came 10 days after the Guardian first revealed footage of Tomlinson being struck and pushed to the ground minutes before he collapsed and died in the City of London.

The New York fund manager who handed the Guardian the video evidence said last night that he felt vindicated by the findings. "Now I'm glad I came forward. It's possible Mr Tomlinson's death would have been swept under the rug otherwise. You needed something incontrovertible. In this case it was the video."

Tomlinson, a 47-year-old newspaper vendor, had been attempting to walk home from work the when he collapsed and died around 7.25pm on 1 April. Moments earlier he had been attacked from behind by a constable from the Met's territorial support group near the Bank of England.

The first postmortem results - which were released by police - said Tomlinson had died of a heart attack. The second postmortem was ordered by the family's legal team and the IPCC after the footage was broadcast.

Last night's developments will place enormous pressure on the IPCC. Initially, the watchdog allowed City of London police to conduct its own inquiry, even though witnesses were coming forward to say they had seen Tomlinson being in contact with police.

The second postmortem was conducted by Dr Nat Cary, who was able to scrutinise video evidence before conducting his examination. In a statement last night, City of London coroners court said Dr Cary had provisionally concluded that internal bleeding was the cause of Tomlinson's death. "Dr Cary's opinion is that the cause of death was abdominal haemorrhage. The cause of the haemorrhage remains to be ascertained.

"Dr Cary accepts that there is evidence of coronary atherosclerosis but states that in his opinion its nature and extent is unlikely to have contributed to the cause of death."

Jules Carey, the lawyer acting for Tomlinson's family, said: "The video footage of the unprovoked and vicious assault on Ian by the police officer would easily justify charges of assault being brought against the officer. The findings of Dr Nat Cary significantly increase the likelihood that the officer will now face the more serious charge of manslaughter."

Last night politicians and campaigners called for the IPCC to conduct its investigation with renewed urgency. The mayor of London, Boris Johnson, said that the watchdog should now come to "a fast and transparent conclusion". The Liberal Democrat home affairs spokesman, Chris Huhne, said: "This is an alarming finding. It suggests that Mr Tomlinson's treatment by the police officer caught on video may have been the final contributing factor in his death. These findings put further pressure on the IPCC to investigate the matter with all urgency."

Neither the IPCC nor City of London police made any mention of the injuries or abdominal blood found by the pathologist Dr Freddy Patel when they released results of the first postmortem. City of London police said only that Tomlinson had "suffered a sudden heart attack while on his way home from work".

Tomlinson's son Paul King said: "We believe we were badly misled by police about the possible role they played in Ian's death. First we were told that there had been no contact with the police, then we were told that he died of a heart attack. Now we know that he was violently assaulted by a police officer and died from internal bleeding. As time goes on we hope that the full truth about how Ian died will be made known."