Russia's embassy in London on Sunday accused Britain of "deliberately" withholding information on probes into the targeting of several Russians on its soil, as the war of words between the two countries continues.
The embassy said it had asked the British Foreign Office for "detailed information" on the investigation into the March 12 murder in London of Russian exile Nikolai Glushkov.
The 68-year-old businessman, who had received political asylum in Britain after being jailed in Russia for money laundering and fraud, died from "compression to the neck", according to a post-mortem.
The death came a week after the nerve agent poisoning of former double agent Sergei Skripal and his daughter Yulia in the English city of Salisbury.
Britain has blamed the attack on Moscow, which has angrily denied involvement.
"Almost a month has passed since Mr Glushkov's death, and like it happened with Sergei and Yulia Skripal, the British side provided no information," the embassy said in a statement.
"Given our numerous requests, the only thing we can suggest is that it is done deliberately."
It added Ambassador Alexander Yakovenko planned to request a meeting with the Commissioner of London's Metropolitan Police for further information.
"For Russia this murder has a criminal as well as political dimension," the statement said.
British Foreign Secretary Boris Johnson wrote on Sunday that "no other government devotes as much time and effort to the business of trying to sabotage or discredit international inquiries" as Russia.
"The essence of a Kremlin cover-up is a cynical attempt to bury awkward facts beneath an avalanche of lies and disinformation," he added.
The attempted killings of the Skripals, who Britain says were targeted with a Soviet-made military-grade nerve agent known as Novichok, has led to the biggest wave of tit-for-tat expulsions of Russian and Western diplomats in decades.
Britain's interior ministry on Friday rejected a visa application by the niece of Sergei Skripal because her application "did not comply with immigration rules".
Viktoria Skripal told Russia's Interfax news agency Saturday that it was denied because Yulia Skripal did "not want to see me".
The Russian embassy in London called the decision "disappointing" and "politically motivated".
The Skripals' health has continued to improve this week, with neither now in critical condition.
Media reports in Britain on Sunday suggested authorities are already planning for their futures.
The Sunday Times said London is in discussions with America about giving the pair new identities and lives in the US to ensure their safety.
The Daily Telegraph reported they could be placed in Britain's witness protection scheme.
Meanwhile, the case has continued to generate criticism of British Labour opposition leader Jeremy Corbyn, who has been more restrained in his blaming of Moscow.
Johnson on Sunday described Corbyn as "the Kremlin's useful idiot" for his apparent scepticism of Russia's culpability over the poisoning.
A Labour spokesman said the foreign secretary had "made a fool of himself and undermined the government" in his handling of the incident.