A former Russian spy and his daughter who were poisoned with military-grade nerve agent in Salisbury over three weeks ago "may never recover fully", the prime minister has disclosed.
Theresa May also told the House of Commons that more than 130 people in the city could potentially have been exposed to the nerve agent.
Her statement to MPs came after the United States and 14 European Union nations expelled dozens of Russian envoys in a coordinated response to the attack.
The PM said: "Sergei and Yulia Skripal remain critically ill in hospital. Sadly, late last week, doctors indicated that their condition is unlikely to change in the near future, and they may never recover fully.
"This shows the utterly barbaric nature of this act, and the dangers that hundreds of innocent citizens in Salisbury could have faced."
She added: "While Public Health England have made clear that the risk to public health is low – and this remains the case – we assess that more than 130 people in Salisbury could have been potentially exposed to this nerve agent.
"More than 50 people were assessed in hospital, with Detective Sergeant Nick Bailey taken seriously ill."
May said Russia had a long record of conducting "state-sponsored assassinations" and that it viewed "some former intelligence officers as legitimate targets for these assassinations".
"We have information indicating that within the last decade, Russia has investigated ways of delivering nerve agents, probably for assassination, and as part of this programme has produced and stockpiled small quantities of Novichoks," she said.
"Clearly, that is in contravention of the chemical weapons convention."
May hit out at Russia for failing to provide an explanation for how the nerve agent came to be used in the UK.
"Instead they have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt, and defiance," she said. "Incredibly, they have deployed at least 21 different arguments about it."