The UK police have always insisted that there is no evidence of foul play in the financier’s death – despite the fact that his name was found on a Russian hitlist and he received a string of threats in his final months – and the Home Office has invoked national security powers to withhold evidence from the inquest.

An independent expert from Kew Gardens told an earlier hearing of Perepilichnyy’s inquest that she had found a substance with the same atomic weight as the plant toxin gelsemium in the dead man’s stomach. However, the same expert told the inquest on Tuesday that further tests could not definitively identify that substance as gelsemium or any other plant toxin, and its origins could not be identified after the police toxicologist threw away most of the contents of Perepilichnyy’s stomach. The inquest is due to hear further evidence from toxicology experts later this week.

British spy agencies have spent years gathering intelligence about top-secret Russian laboratories where teams of state scientists are tasked with developing poisons capable of killing without leaving a trace.

Medynska’s secret assignation with Perepilichnyy in Paris was discovered by French police who opened a murder investigation after the earlier testimony that traces of plant poison may have been found in his stomach. French detectives seized hotel records showing that Perepilichnyy was joined at the five-star Bristol hotel for two nights by the young Ukrainian woman, who claimed online to have trained as a fashion designer in Moscow. The records showed that the pair stayed for two nights, ordering the hotel’s “romance pack”, and checked out on 10 November. It was later that same day that Perepilichnyy died.

French detectives told BuzzFeed News last year that Medynska had been identified as a “very important” person to their inquiry because “she’s one of the last persons to have seen Mr Perepilichnyy alive” – but they said the British authorities had repeatedly stonewalled their attempts to investigate the financier’s death, meaning they had not been able to interview her.

Two senior French law enforcement sources have now told BuzzFeed News that UK officials suddenly began cooperating and made efforts to secure Medynska's testimony following the revelations by BuzzFeed News last summer. The attitude of the British authorities had “completely changed”, one source said on condition of anonymity, and it appeared that officials were now “under pressure to investigate the case deeply”.

Perepilichnyy’s Paris visit was shrouded in mystery before BuzzFeed News obtained details of the French police file and tracked Medynska down to an opulent penthouse apartment on the glitzy Avenue Victor Hugo in Paris.

None of Perepilichnyy’s family, friends, or business associates knew what he had been doing in the French capital, and the only clues were that he had taken a Eurostar train on 6 November 2012 and his credit card statements suggest he had booked into two hotels simultaneously. He returned on 10 November with a €1,200 receipt from the Prada store on the Champs-Élysées but nothing to show for the purchase.