There's now a national security angle to the Russian whistleblower who mysteriously dropped dead while jogging

The long-awaited inquest into the mysterious death of a Russian whistleblower has been delayed because the UK government is trying to stop certain documents related to the case being made public.

Alexander Perepilichny died while jogging near his home in Surrey four years ago.

Traces of a deadly chemical compound, which is found in a poisonous plant known as heartbreak glass, were found in his stomach, according to evidence in a pre-inquest hearing.

Prior to his sudden death, Perepilichny had been assisting Hermitage Capital, an investment fund, in exposing an alleged Russian laundering operation worth millions of dollars, according to the Financial Times.

The inquest into the whistleblower's death was due to get under way at a coroners' court in Woking, Surrey, next Monday. However, it has been pushed back until at least March next year due to a legal wrestling match over documents which the government argues would undermine national security if made public.

Richard Travers, the coroner who was set to oversee the inquest, said at a hearing on Tuesday that he had "no choice" but to delay proceedings because the government wanted to secure Public Interest Immunity status (PII) for sensitive documents directly related to the investigation into Perepilichny's death.

PPI status is a rarely-used legal apparatus which is applied in cases where the disclosure of sensitive documents would pose a clear threat to national security. Henrietta Hill, a barrister for Hermitage Capital, claimed the documents in question were "plainly relevant" to the "central question" of whether Perepilichny was murdered.

It is not known what information the documents contain. However, the fact the government has lodged a request to have them legally protected suggests they have massive implications for the UK's national security.

In earlier hearings, legal representatives for Hermitage alleged there was "potential parallels" between the mysterious death of Perepilichny and the death of former Russian spy Alexander Litvinenko. A public inquiry held earlier this year concluded that the latter died in 2006 after being poisoned in an operation ordered by the Russian state.

Perepilichny and Litvinenko's cases are also similar because both were involved in major efforts to expose corruption among Russian elites immediately before their deaths. The latter had been due to give evidence to a Spanish prosecutor relating to Vladimir Putin's links to organised crime just a week after he was poisoned.