A chemical linked to a rare poisonous plant was found in the stomach of a wealthy Russian businessman who died unexpectedly in the UK three years ago, an inquest has heard.

Alexander Perepilichny, 44, collapsed outside his mansion on a luxury private estate near London, on November 10, 2012.

A lawyer for Perepilichny's life insurance company, Bob Moxon Browne, told the pre-inquest hearing into the death that test results had revealed a chemical that could be traced to the poisonous plant species Gelsemium.

"Once you have knocked out man-made analogues then you are bound to conclude Mr Perepilichny ingested Gelsemium on the day of his death," he said.

"Then, given that it only grows in China and is a known weapon of assassination by Chinese and Russian contract killers, why was it in his stomach?"

A spokeswoman for the Royal Botanic Gardens in London said the institution, "has supplied its expertise in plant chemistry to assist the coroner's inquest."

But she declined to comment further "due to the sensitive nature of this work" until the inquest is completed.

The most toxic variety of the Gelsemium plant, Gelsemium elegans, is also known as "heartbreak grass" and early symptoms of ingestion include dizziness, nausea and convulsions.

Higher doses can cause paralysis of the spinal cord, a loss of muscular power and eventually asphyxia.

The Moscow grave of anti-corruption lawyer Sergei Magnitsky ( AFP: Andrey Smirnov )

Perepilichny was reportedly providing evidence to an investigation into an alleged money laundering scheme involving Russian tax officials at the time of his death.

The scheme was thought to be linked to the death of Russian anti-corruption lawyer Sergei Magnitsky in 2009.

Before his death, Magnitsky claimed to have uncovered the plan used by police officials to reclaim about $235 million in taxes paid by US investment firm, Hermitage Capital - once Russia's largest Western investment fund.

As a result, officials charged Magnitsky with fraud and held him in a notoriously squalid Russian prison, where he died at the age of 37.

The inquest into Perepilichny's death is expected to be completed later this year.

AFP