The statement from the City of London Coroners Court overturns the initial assessment that the newspaper seller died of natural causes.
Mr Tomlinson was hit and pushed over by a police officer on 1 April.
The Independent Police Complaints Commission (IPCC) is investigating Mr Tomlinson's death.
The Coroner's statement said the second post-mortem's conclusions were provisional.
In its statement, the Coroner's Court said that the inquest had looked at the first post-mortem carried out after Mr Tomlinson collapsed and died on the evening of 1 April.
That examination, carried out by Dr Freddy Patel, concluded that Mr Tomlinson had diseased heart and liver and a substantial amount of blood in the abdominal cavity.
"His provisional interpretation of his findings was that the cause of death was coronary artery disease," said the statement.
"A subsequent post-mortem examination was conducted by another consultant forensic pathologist, Dr Nat Cary, instructed by the IPCC and by solicitors acting for the family of the late Mr Tomlinson.
"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."
The statement concluded that both the opinions remained provisional and subject to further investigations and tests.