Seafood is now the "number one suspect" in the death of wealthy Russian whistle-blower Alexander Perepilichnyy, who died after collapsing while jogging near his Weybridge home.

He spent the night before the collapse in November 2012 in Paris, dining out with his ex-model mistress, Elmira Medynska.

She told an inquest at the Old Bailey on Wednesday, April 11, he sent back “bad” tempura prawn and ate either sushi or sashimi, and then vomited repeatedly back at the hotel.

Ms Medynska said: "I think maybe he vomited because it was not good food in the restaurant.”

But a "malignant" poisoner at the smart Japanese restaurant could also be to blame, it was claimed.

The married Mr Perepilichnyy was 44 when he collapsed, and his death sparked allegations of a “reprisal killing” after it emerged he had helped Swiss authorities in a major corruption investigation.

At a fresh inquest coroner Nicholas Hilliard QC is examining how he died, whether he was poisoned and who might have had a motive for murder.

Cardiologist Dr Peter Wilmshurst told the inquest histamine or scombroid poisoning could result from eating long distance fish, such as salmon, tuna and mackerel.

Symptoms include rashes, hives cramps, vomiting, a "peppery taste" and itching.

Dr Wilmshurst, who had suffered fish poisoning himself, said: "It's rarely fatal. There are cases of people who have died of it."

On the effect on the heart, he said: "It can do all sorts of things to the heart. It often causes the heart rate to go fast. Histamine has direct effects on the heart."

He said there was a "more than 50 per cent" chance it was poisoning, if he ate fish at the Japanese restaurant.

Asked if that contributed to his death, he said: "If one accepts he had scombroid fish poisoning that night then dies the next day having had a condition 18 hours earlier – if you cannot find any other reason, that becomes the number one suspect."

But he added: "The big problem is there are so many unknowns."