Britain has rejected a Russian call for it to be involved in the investigation into the nerve agent attack in Salisbury.
Addressing an extraordinary meeting of the Organisation for the Prohibition of Chemical Weapons (OPCW) in The Hague, UK ambassador John Foggo said that the demand was a sign that Moscow was "nervous" of what the probe will find.
Mr Foggo accused Russia of showing "disdain" for the independence of the international body, which is conducting tests on samples of the nerve agent used in the March 4 attack on ex-spy Sergei Skripal and his daughter Yulia.
Moscow called the meeting of the OPCW's executive council to insist that its experts must be involved in the testing programme and to demand a joint UK/Russian investigation into what happened at Salisbury.
But Mr Foggo dismissed the move as "cynical", telling the meeting: "The work of the Technical Secretariat must remain impartial.
Mr Foggo said that Russia's statements displayed a "wilful ignorance" of the Chemical Weapons Convention and "disdain for the independence and competence" of the OPCW's Technical Secretariat.
"We will not agree to Russia's demand to conduct a joint investigation into the attack in Salisbury because the UK - supported by many other countries - has assessed that it is highly likely that the Russian State is responsible for this attack, and that there is no plausible alternative explanation," he said.
"There is no requirement in the Chemical Weapons Convention for a victim to engage the likely perpetrator in a joint investigation. To do so would be perverse."
The UK was backed by the European Union, which reaffirmed its support for Britain's demand for answers from Russia on how the Novichok agent which it developed came to be used in Salisbury.
Speaking on behalf of the EU, Bulgarian ambassador Krassimir Kostov said: "We have full confidence in the UK investigation and laud UK's collaboration with the OPCW Technical Secretariat, in full compliance with the convention."
But the Russian Embassy in the Netherlands said that Moscow's position had won the support of 14 state parties in the OPCW meeting.
"We consider it's necessary to ensure that this problem is solved within the international legal framework using the full potential of the Chemical Weapons Convention," said the embassy.
Tension between Moscow and London has risen a notch after the head of the Porton Down military research facility said scientists had not verified Russia as the source of the substance used in the attack on the Skripals.
Vladimir Putin seized on the comments from the chief executive of the Government's Defence Science and Technology Laboratory (DSTL), Gary Aitkenhead, as he accused the UK of launching an "anti-Russian campaign".
Russia has flatly denied UK claims that it was to blame for the March 4 attack, with foreign intelligence service director Sergei Naryshkin even claiming it was staged by the UK and US as a "provocation".
Mr Naryshkin told a global security conference in Moscow: "Even as far as the Skripal case goes - which is a grotesque provocation rudely staged by the British and US intelligence agencies - some European countries are in no hurry to follow London and Washington, preferring to sort the situation out."
Mr Foggo said that, rather than providing a "substantive response" to the UK's questions, Moscow had indulged in a series of at least 24 "shameless and preposterous" counter-narratives, many of which contradict one another.
He said Russia had shown "contempt" for the victims of the Khan Sheikhoun chemical weapon attack in Syria by calling the OPCW meeting on the first anniversary of the atrocity.
Moscow continues to demand that Britain hand over samples of the substance found in Salisbury so it can carry out its own checks.
But a Foreign Office spokesman said there was "no requirement in the chemical weapons convention for the victim of a chemical weapons attack to engage in a joint investigation with the likely perpetrator".
The spokesman described Wednesday's meeting in The Hague as "yet another diversionary tactic, intended to undermine the work of the OPCW in reaching a conclusion".
He said: "Russia has called this meeting to undermine the work of the OPCW which, fully in accordance with the chemical weapons convention, is providing the UK with technical assistance and evaluation through independent analysis of samples from the Salisbury attack."