British spy agencies may be ordered to hand over intelligence exposed by BuzzFeed News that Alexander Perepilichnyy was likely assassinated at the behest of Vladimir Putin.

BuzzFeed News revealed on Monday that US spy agencies have handed their British counterparts high-grade intelligence that Perepilichnyy was likely “assassinated on direct orders from Putin or people close to him” after blowing the whistle on a vast crime by Russian government officials. Yet police said Perepilichnyy had died naturally of a heart attack.

Lawyers challenging the police case have now asked the judge hearing his inquest to order MI6 to disclose the intelligence MI6 received.

Judge Nicholas Hilliard QC said on Wednesday that he would “carefully” consider the lawyers’ request. If he finds that Britain’s secret services have withheld evidence relevant to Perepilichnyy’s death, Hilliard would likely need to suspend the inquest, which does not have the powers to consider classified information. Only a public inquiry, like the one that found that Putin had “probably” ordered the murder of defector and former Russian spy Alexander Litvinenko, could entertain such evidence.

Theresa May is facing growing calls from MPs, lawyers, and campaigners to respond urgently to the revelations. A Conservative MP urged the prime minister to “move quickly to reassure the public,” while the prosecutor who headed the case against Litvinenko’s killers demanded she reverse her “supine” response to the “real suspicions” that the Kremlin had claimed another victim on UK soil.

Perepilichnyy collapsed and died while out jogging near his home in the exclusive gated community of St George’s Hill in Surrey after returning from a mysterious trip to Paris in November 2012. The financier had been receiving threats ever since he came forward to expose a $230 million tax fraud by corrupt Russian government officials. Surrey police declared "there were no suspicious circumstances surrounding his death”, but two years later a plant expert at Kew Gardens told the inquest she had identified traces of a deadly poison in his stomach.

Police still insist that there is no evidence of foul play, and May’s government has invoked national security powers to withhold evidence from the inquest into the cause of his death. But BuzzFeed News revealed that a highly classified report on Russian state assassinations compiled for the US Congress by the Office of the Director of National Intelligence last year asserts with “high confidence” that Perepilichnyy’s murder was sanctioned by Russian President Vladimir Putin, according to two US intelligence officials. MI6 officers who agreed with this assessment were apparently silenced, a serving US intelligence official said, because “their efforts to publicly declare that Perepilichnyy was assassinated on British soil had deeper political implications”. An ODNI spokesperson said the agency had “no comment beyond confirming that we prepared the report for Congress”.

French police are treating Perepilichnyy’s death as a suspected assassination, but say they have been repeatedly stonewalled by their British counterparts. Perepilichnyy travelled to Paris before his death for a secret assignation with a 22-year-old Ukrainian woman named Elmira Medynska. Medynska still lives in Paris and gave an exclusive interview to BuzzFeed News. But police never spoke to her, and she is not expected to testify at the coroner’s inquest.