The UK will expel 23 Russian diplomats after Moscow refused to explain how a Russian-made nerve agent was used on a former spy in Salisbury, the PM says.
Theresa May said the diplomats, who have a week to leave, were identified as "undeclared intelligence officers".
The UK later told the UN Security Council that Russia had used "a weapon so horrific that it is banned in war" in a "peaceful" British city.
Russia denies attempted murder and says it will respond appropriately.
Mrs May also revoked an invitation to Russia’s foreign minister, and said the Royal Family would not attend the Fifa World Cup in Russia later this year.
Addressing the UN Security Council, Britain’s deputy UN ambassador, Jonathan Allen, accused Russia of breaking its obligations under the Convention on the Prohibition of Chemical Weapons.
He said he had heard the threats from Russia but that the UK would not be deterred.
"We will stand by the values which are shared by the overwhelming majority of those in this council in this United Nations and we ask you today, to stand by us," he added.
In response the Russian ambassador to the UN, Vasily Nebenzya, denied Moscow’s involvement in the attack and demanded "material proof" from Britain to support its charge.
He said: "We were given an ultimatum and requested in 24 hours to admit that we committed a crime. In other words, confess.
"We do not speak the language of ultimatums. We do not use that language with anyone. And we will not allow to be spoken to in that language either."
Mr Allen said the UK government has asked the Organisation for the Prohibition of Chemical Weapons, an independent watchdog, to verify its identification of the substance used in Salisbury.
The US ambassador Nikki Haley said Washington stood in "absolute solidarity" with Britain, citing the "special relationship" between the two countries and saying the US would "always be there" for the UK.