British Foreign Minister Boris Johnson said on Monday that Russian denials of responsibility over a nerve agent attack on a former Russian double agent in England were "increasingly absurd".
Johnson, who briefed fellow European Union ministers in Brussels on Monday, also won renewed support from the bloc.
"The Russian denial is increasingly absurd," Johnson told reporters as he arrived for the regular monthly meeting. "This is a classic Russian strategy ... They're not fooling anybody anymore."
Russia denies any involvement in the attempted murder of Sergei Skripal and his daughter Yulia, in what was the first known offensive use of nerve gas in Europe since World War Two.
Moscow on Saturday announced the expulsion of 23 British diplomats in tat response to Britain's decision last week to expel the same number of Russian diplomats from London.
On Sunday, Johnson accused Russia of stockpiling the deadly Soviet-era nerve agent Novichok used to poison the Skripals, a charge Moscow denies. They were found unconscious on a bench in the English city of Salisbury on March 4 and remain in a critical condition in hospital.
In a joint statement, EU foreign ministers said Monday they took "extremely seriously" Britain's claim that Moscow was behind the attack.
"The European Union expresses its unqualified solidarity with the UK and its support, including for the UK's efforts to bring those responsible for this crime to justice," they said.
But Kremlin spokesman Dmitry Peskov warned Britain to prove Russia's involvement or apologize.
"Sooner or later these unsubstantiated allegations will have to be answered for: either backed up with the appropriate evidence or apologized for," he said.
Inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW) have arrived in Britain to begin an investigation into the March 4 attack, a source at the organization said on Monday.
Britain has asked the chemical weapons body based in The Hague to run independent tests on samples at its laboratories, said the diplomatic source, who spoke on condition of anonymity.
