A Russian whistleblower could have been poisoned with sorrel soup, but key evidence about his last meal was "flushed away" hours after his death, a court heard.
Alexander Perepilichnyy, 44, died after collapsing while running near his home in Weybridge, Surrey, in November 2012.
The businessman's death was originally attributed to natural causes, but traces of a chemical that can be found in the poisonous plant Gelsemium elegans were later found in his stomach.
A pre-inquest has heard that, before his death, Mr Perepilichnyy was helping a specialist investment firm uncover a 230 million US dollar (£150 million) Russian money-laundering operation.
Hermitage Capital Management has previously claimed that Mr Perepilichnyy could have been deliberately killed for helping it uncover the scam involving Russian officials.
At a pre-inquest hearing at the Old Bailey, a lawyer suggested the victim may have dined on a popular Russian dish based on the sorrel herb, which could have been switched.
Bob Moxon Browne, QC, for Legal and General Assurance Society, queried why no one appeared to have asked Mr Perepilichnyy's widow what he had for lunch that day.
He said: "The contents of Mr Perepilichnyy's stomach were flushed away very shortly after his death. There is no bag of stomach contents. There is a quantity of material that was subsequently retrieved from the stomach cavity."
Tests had shown a "suspect compound" that matched the atomic weight of a "vegetable poison", he said.
Mr Moxon Browne said: "If he was murdered, it does seem likely he was poisoned rather any other method of bringing about his death."
He said the likely poison was vegetable rather than irradiation or a heavy metal.
He said: "It is almost incredible a fact no statements have been taken by police from the widow, who was with him that day and had lunch with him."
He said there was a "rumour" he had soup containing sorrel, which is a component of a popular Russian dish, but tests did not identify the herb in his stomach contents.
He said the examination was either "not fit for purpose" or there was a "possibility somebody had substituted another vegetable matter for sorrel".
The court heard of evidence Mr Perepilichnyy had received threats by phone from an organised crime group and had taken out "multiple" life insurance policies before his death.
Henrietta Hill QC, for Hermitage, called for a "wider" investigation and said on the day of his death, his daughter had spent a "significant" amount of time with her father.
Ms Hill said: "There is an issue why Mr Perepilichnyy had so much life insurance. It has been suggested at one point he was advised to take out multiple policies by his bank manager."
Last November, Home Secretary Amber Rudd won a High Court order preventing the disclosure of ''sensitive material'' at the inquest.
After coroner Nicholas Hilliard QC had reviewed the secret material, a "form of words" was agreed by the coroner and Government.
It stated: "Nothing in the material that was subject to the Public Interest Immunity (PII) application materially assists the coroner in answering the question of how Alexander Perepilichnyy died.
"Nothing in the material alters the decision on scope."
The coroner said: "The form of words represents my view at this stage about the significance of sensitive material which I have reviewed."
Ms Hill said it "came as something of a surprise" that none of the material had "materially assisted" the inquest.
But Peter Skelton QC, counsel for the inquest, said: "Relevance is a matter for the coroner. A piece of evidence may be relevant, but it may not in the overall scheme of the inquest be significant."
The coroner set a three to four-week full inquest for June 5 at a London court, but said he was proposing to deliver his conclusions in Surrey.
Mr Moxon Browne suggested Special Branch had been "keeping tabs" on the victim's movements as he frequently travelled by train in the months before his death.
He said: "The question we will be wanting to ask - how does it come about they knew exactly what he was doing but Surrey Police were unable to identify him for three to four weeks after his death."
He called for previous rulings in the case to be made public, including the decision not to have an inquest jury and whether the deceased had or had not been in contact with British intelligence.
Fiona Barton QC, for Surrey Police, said no "identifiable toxin" was found in the body.