British ambassador Karen Pierce said she would take no lectures from Moscow and would “stick to the facts” in a heated emergency session of the UN Security Council.
She compared Moscow’s demand to be included in the investigation of Sergei and Yulia Skripal’s poisoning to “an arsonist trying to investigate his own fire”.
The meeting on Thursday night was called by Russia, with its UN ambassador Vasily Nebenzya seeking to capitalise on days of confusion over the attempted murders.
He opened the session by asking why viewers of the British TV programme Midsomer Murders would believe Russia chose “the most risky and dangerous method” to kill two of its citizens.
“They all know hundreds of very clever ways of killing someone,” he declared.
Mr Nebenzya then praised the chief executive of Porton Down, who told Sky News on Tuesday that scientists had not yet discovered the “precise source” of the nerve agent used in Wiltshire last month.
“He did not pay tribute to the guesswork put forward by British security services,” the ambassador said, adding that the novichok substance identified “is obviously not copyrighted by Russia”.
Turning the heat up, Mr Nebenzya told Britain it was “playing with fire” – and warned: “You’ll be sorry.”
He then likened the media to a “psychotropic substance” and the Western coalition that expelled diplomats to Nazi propagandist Joseph Goebbels.
Mr Nebenzya’s argument focused on asking questions, insisting that Russia only wanted “to get to the bottom of the story”.
He concluded: “The investigation is far from being over, in fact it has only just begun.”
Hitting back, Ms Pierce said British accusations “stand up to any scrutiny”.
She defended Foreign Secretary Boris Johnson, saying there was “no contradiction” in saying Porton Down officials had “no doubt” when asked why he was sure the nerve agent originated in Moscow.
“The Foreign Secretary was making clear Porton Down is sure the nerve agent was a novichok,” she said.
“The threats to the chemical weapons convention, in Syria, in Malaysia and now the UK, place a very serious challenge to the non-proliferation regime that this organisation has constructed in response to the terrible events of the past.
“Russia is playing fast and loose with the institutions that protect us.”
US ambassador Kelley Currie followed up by attacking Russia’s comparison of Western states to Goebbels, saying it was “not a tactic appropriate for this body”.
She also accused Mr Nebenzya of “using this chamber to undermine the truth”.
The emergency session came hours after Yulia Skripal issued her first statement on the attack, saying her “strength is growing daily”.