The family of a Russian whistleblower have accused "well funded forces" of deliberately manipulating the media to put pressure on a coroner to pursue a theory that he was assassinated on the orders of the Kremlin.
Alexander Perepilichnyy, 44, collapsed and died while running near his home in Weybridge, Surrey, in November 2012.
The businessman's death was originally attributed to natural causes, but an inquest at the Old Bailey before coroner Nicholas Hilliard QC is looking into whether he was poisoned and who had a motive to murder him.
At the time of his death, Mr Perepilichnyy had been helping specialist investment firm Hermitage Capital Management uncover a 230 million US dollar (£150 million) Russian money-laundering operation.
Hermitage has claimed that Mr Perepilichnyy could have been killed deliberately for helping it uncover the scam involving Russian officials.
On Friday, William Browder, the co-founder and chief executive officer of Hermitage, had attacked the Surrey Police investigation into Mr Perepilichnyy's death and claimed officers ignored his warnings.
He said: "It became clear to us there was some type of blockade, either intentional or based on negligence, by the police and the only way you could get an acknowledgement was to go to the press, so we went to the press.
"The police were refusing to investigate. If you think a guy has been killed and the police refuse to investigate of course you are trying to influence the police to investigate and influence the police to do their job."
Late on Monday, online news outlet BuzzFeed published an article claiming US intelligence had passed information to British counterparts suggesting Mr Perepilichnyy was assassinated.
In the wake of the report, Henrietta Hill QC, for Hermitage, called for the coroner to ask US and British authorities to hand over the material.
The news report claimed "high grade" intelligence led to a report prepared for Congress by the Office of the Director of National Intelligence (ODNI) with CIA and National Security Agency assistance in which it was asserted with "high confidence" the businessman was killed on the orders of the Kremlin.
Ms Hill said: "It is said not only that the report was provided to British intelligence agencies, but that MI6 officers all agreed with this conclusion and were apparently silenced, it is reported.
"This is on the face of it highly pertinent information. Intelligence from several different officials within US intelligence. It is potentially highly significant evidence that has been referred to."
She said a BBC Radio 4 report on Tuesday suggested the information had been "independently verified".
Robert Moxon Browne, for Legal and General which provided Mr Perepilichnyy with a large life insurance policy, supported the application.
But John Beggs QC, representing widow Tatiana Perepilichnaya, questioned who was really behind the BuzzFeed story and why it was published in the middle of the inquest.
He told the coroner: "If you are going to make any request, and I can understand why you might consider you needed to do so, you might be interested in asking BuzzFeed when did they receive these stories, when were they placed with BuzzFeed and who placed them?
"We have our theories as to the timing. We observe Mr Browder's evidence got very little publicity no doubt due to the General Election and shortly after an alternative route to publicity was secured by BuzzFeed.
"There is a possibility of cogent circumstantial evidence there may be well-funded forces out there deliberately manipulating the media.
"We have heard cogent rumours that is precisely what is going on and you are being placed under pressure by skillful media manipulation."
Mr Beggs said he had no notice of the application to demand the secret material from US and UK intelligence agencies.
He said: "On the current state of evidence a respectable viewpoint is that there is zero evidence of third party involvement yet there is credible evidence of channelopathy being the cause of death.
"But there will not be any newspapers publishing 'Wealthy Russian dies through tragic genetic mutation'. It tends not to make headlines."
Coroner Mr Hilliard responded: "I shall consider everything you have said."
Last November, Home Secretary Amber Rudd won a High Court order preventing the disclosure of ''sensitive material'' at the inquest.
But after reviewing all the material the coroner said he found nothing that "materially assists the coroner in answering the question of how Alexander Perepilichnyy died".