Britain is to kick out 23 Russian diplomats, the biggest such expulsion since the Cold War, over a chemical attack on a former Russian double agent in England that Prime Minister Theresa May blamed on Moscow.
May pointed the finger firmly at Russian President Vladimir Putin on Wednesday as she outlined retaliatory measures in parliament.
Russia denies any involvement in the attack on ex-spy Sergei Skripal and his daughter Yulia, who have been critical in hospital since they were found unconscious on March 4 on a bench in the city of Salisbury.
May announced the potential freezing of Russian state assets that pose a security threat, new laws to counter hostile state activity and a downgrading of Britain's attendance at the soccer World Cup in Russia this summer.
She had given Moscow until midnight on Tuesday to explain how the Soviet-made Novichok nerve agent came to be deployed on the streets of Salisbury, saying either the Russian state was responsible or had lost control of a stock of the substance.
"Their response demonstrated complete disdain for the gravity of these events," May said in her statement to parliament.
The Russian Foreign Ministry said Moscow would swiftly retaliate against the British measures which had been undertaken for "short-sighted political ends".
Russia's Ambassador to the United Nations, Vassily Nebenzia, repeated Moscow's denial that it had anything to do with the poisoning and called for proof of its involvement.
Britain, which has received statements of support from the United States, the European Union and NATO, has said it would seek to coordinate an international response to the attack.
The White House issued a statement saying it shared Britain's assessment that Russia was responsible and supported May's decision to expel the diplomats "as a just response."
At the United Nations, U.S. Ambassador Nikki Haley blamed Russia for the attack and urged the Security Council to take "immediate, concrete measures to address this now."
German Chancellor Angela Merkel said she took the British findings seriously and there would be a common European stance, although "we must still talk responsibly" with Russia despite differences of opinion.
A French government spokesman said Paris was awaiting proof before deciding if it would act in solidarity with London.
Skripal betrayed dozens of Russian agents to Britain before being arrested in Moscow and later jailed in 2006. He was freed under a spy swap deal in 2010 and took refuge in Britain.
The attack on him was likened in Britain to the killing of ex-KGB agent Alexander Litvinenko, a critic of Putin, who died in London in 2006 after drinking green tea laced with radioactive polonium 210.
Britain's response to the Litvinenko affair, which included the expulsion of four Russian diplomats, was criticised domestically as too weak, and many in British politics and media have called for a much tougher response to the Skripal attack.
May said the 23 diplomats, identified as undeclared intelligence officers, had one week to leave and Russian intelligence capabilities in Britain would be damaged for years.
May also said Britain would revoke an invitation to Russian Foreign Minister Sergei Lavrov to visit and suspend planned high level bilateral contacts between London and Moscow.
On the soccer World Cup, which Russia is hosting in June and July, May said no ministers or members of the British royal family would attend.
But unlike when the United States and European Union imposed sanctions on Russia in response to its annexation of Crimea, May did not name Russian people or companies that would be specifically targeted by sanctions.