The head of the global chemical weapons watchdog agency has rejected Russian claims that traces of a second nerve agent were discovered in Salisbury where a former Russian spy and his daughter were poisoned.
The UK blames Russia for the attack, which it says was carried out by smearing a Soviet-developed nerve agent known as Novichok on a door handle at Sergei Skripal's house in Salisbury.
Moscow denies involvement in the incident in which Mr Skripal's daughter Yulia was also poisoned.
Russian foreign minister Sergei Lavrov said Moscow received confidential information from the laboratory in Spiez, Switzerland, that analysed samples from the site of the March 4 poisoning.
He said the analysis, done at the request of the Organisation for the Prohibition of Chemical Weapons, indicated that samples contained BZ nerve agent and its precursor.
He said BZ was part of the chemical arsenals of the US, Britain and other Nato countries, while the Soviet Union and Russia never developed the agent.
OPCW director-general Ahmet Uzumcu told a meeting of the organisation's executive council that a BZ precursor known as 3Q "was contained in the control sample prepared by the OPCW Lab in accordance with the existing quality control procedures".
He added "it has nothing to do with the samples collected by the OPCW team in Salisbury".
Britain's representative to the OPCW, Ambassador Peter Wilson, said the Russian foreign minister's comments were a breach of the treaty outlawing chemical weapons.
"The thing for me that was particularly alarming about Lavrov's statement is, first of all, the OPCW goes to enormous lengths to make sure that the identity of laboratories is confidential and, second of all, either the Russians are hacking the laboratories or they are making stuff up," he said.
"Either way, that is a violation of the confidentiality of the Chemical Weapons Convention."
In a summary of its report last week, the OPCW did not name Novichok as the nerve agent used but it confirmed "the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury".
Mr Wilson told reporters the OPCW "confirmed that they found what we found, and that is a Novichok".
Russia's representative to the OPCW, Ambassador Alexander Shulgin, repeated Moscow's denials and accused Britain of a string of lies.
"For now, I will only say one thing: the claim that the Technical Secretariat confirmed that this chemical points to its Russian origin is an outright lie," he said in a statement posted on his embassy's website.
"The report itself does not say a single word about the name 'Novichok;' the CWC simply does not contain such a concept."
Mr Wilson told the meeting that London continues to believe evidence points to Russian involvement in the attempted assassination.
"We believe that only Russia had the technical means, operational experience and motive to target the Skripals," Mr Wilson said.
Mr Wilson warned the Chemical Weapons Convention was being undermined by a growing use of nerve agents and other poisons, mentioning the 2017 assassination in Malaysia of North Korean leader Kim Jong Un's estranged half brother, in addition to the Salisbury attack and the use of poison gas in Syria and Iraq.
"It is being continually violated," Mr Wilson told reporters.
He said the convention would be strengthened if all nations fully declared any stockpiles they still have.
Member states are supposed to declare all their chemical weapons stocks upon joining the OPCW and destroy them.
The OPCW and Russia last year celebrated the destruction of the country's final declared stocks.
"Russia clearly has chemical weapons they are not declaring and they need to do that," Mr Wilson said.