Evidence has emerged for the first time that a millionaire Russian businessman found dead outside his Surrey home may have been poisoned by assassins - despite the insistence of police that his death was not suspicious.

Alexander Perepilichnyy collapsed and died outside his home on a select estate in Weybridge in November 2012 shortly before he was due to give evidence as a whistleblower in an investigation into an alleged $230m (£146m) fraud which implicated Russian tax officials in a conspiracy with organised criminals. He had been told he was on a mafia hit list.

The news of Mr Perepilichnyy's death was first reported by The Independent, sparking interest in the case from around the world.

The sudden death of a hitherto healthy man in the depths of the Stockbroker Belt led to intense speculation that the 44-year-old was the target of contract killers from the Russian mafia. But Surrey Police insisted within 12 months of the death that extensive toxicology testing and other investigations showed the death was not suspicious and there had been "no third-party involvement".

But at a pre-inquest hearing it emerged that fresh testing by a leading poisons expert has revealed the presence of a chemical in a sample of the stomach contents of Mr Perepilichnyy which is strongly associated with a lethal plant toxin known to be used by Russian contract killers.

Lawyers for police acknowledged in a hearing at Surrey Coroner's Court in Woking that the presence of the chemical "ion" was a "cause for very serious concern".

Further tests are now being urgently carried out to establish whether the chemical "calling card" in the stomach contents can be used to show that Mr Perepilichnyy must have swallowed a deadly plant poison shortly before his death.

The court heard that the substance was extremely rare in nature and could only be derived naturally from five sources - all of them forms of the poisonous plant Gelsemium, otherwise known as "heartbreak grass" and a known tool of assassins from Russia and China, where the most toxic version of the shrub - Gelsemium elegans - grows on remote hillsides.

The fast-acting poison is seldom used and the last known case of its use was in 2011 to murder a Chinese billionaire, Long Liyuan, who is thought to have ingested the toxin while eating a dish of cat stew.

The Independent understands that the presence of the suspect substance emerged last year after testing of samples of stomach contents, blood and urine by Professor Monique Simmonds, a leading plant poisons experts at the Royal Botanic Gardens in Kew, west London.

The full inquest hearing into Mr Perepilichnyy's death had been due to begin on 18 May but Surrey Coroner Richard Travers has now ordered the proceedings be delayed until September to allow Prof Simmonds to carry out further testing to establish whether the stomach chemical can be shown to have come from Gelsemium elegans or a non-toxic artificial "analogue".

Dijen Basu, for Surrey Police, told the court: "This is a really unusual case where there is cause for very serious concern. We have a suspect substance in the stomach."

Any evidence that Mr Perepilichnyy was indeed murdered - potentially by assassins sent by his enemies in the Russian mafia - would have echoes with the poisoning of former Russian intelligence officer Alexander Litvinenko and raise uncomfortable questions for Surrey Police about the thoroughness of its original investigation.

Critics claimed that the force was slow to take seriously claims that Mr Perepilichnyy may been killed after he was found face down on the verge in a jogging suit outside his home on a gated estate in Weybridge whose residents have included other Russian oligarchs.

In July 2013, the force announced that it had found no evidence of foul play in Mr Perepilichnyy's death and said it was handing over the investigation to the coroner. Detective Chief Inspector Ian Pollard, the officer in charge of the investigation, said: "I am satisfied that following extensive enquiries, including a post mortem examination carried out by a Home Office pathologist and a full and detailed range of toxicology tests, there is no evidence to suggest that there was any third party involvement in Mr Perepilichnyy's death."

It is believed that investigators reached the conclusion that Mr Perepilichnyy had most likely died from natural causes, possibly due to a previously undiagnosed heart condition.

The 44-year-old businessman arrived in Britain in 2010 and came forward as a whistleblower in the Sergei Magnitsky affair - the Russian lawyer who died in custody after exposing another multi-million dollar fraud involving corrupt officials and a mafia money laundering network.

Mr Perepilichnyy, who took out multiple life insurance policies believed to be worth a seven-figure sum shortly before his death, had been due to testify to the Swiss authorities after he exposed a separate alleged fraud involving a Moscow tax official and her husband who were accused of hiding the proceeds in Swiss bank accounts. The businessman reportedly said he had received death threats from Russia in connection with the case.

Mr Travers has previously ruled that he will not hear evidence concerning Mr Perepilichnyy's background at any inquest but he said the situation would be "plainly different" were evidence of poisoning to emerge and the scope of any hearing may have to change.

Lawyers for the insurance companies with which the businessman took out his policies said it was now vital that the issue of the potential poisoning be resolved.