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”.
She also revoked an invitation to Russia’s foreign minister, and said the Royal Family would not attend the Fifa World Cup later this year.
Russia denies being involved in the attempted murder of Sergei Skripal.
Former spy Mr Skripal, 66, and his daughter, Yulia Skripal, 33, remain critically ill in hospital after being found slumped on a bench on 4 March.
Det Sgt Nick Bailey also fell ill responding to the incident, and is in a serious but stable condition, but is thought to be improving.
Moscow refused to meet Mrs May’s midnight deadline to co-operate in the case, prompting Mrs May to announce a series of measures intended to send a “clear message” to Russia.
Among them are:
- The expulsion of 23 diplomats – who have one week to leave
- Increased checks on private flights, customs and freight
- The freezing of Russian state assets where there is evidence they may be used to threaten the life or property of UK nationals or residents
- Ministers and Royal Family to boycott the Fifa World Cup in Russia later this year
- The suspension of all planned high level bi-lateral contacts between the UK and Russia
Mrs May told MPs that Russia had provided “no explanation” as to how the nerve agent came to be used in the UK, describing Moscow’s response as one of “sarcasm, contempt and defiance”.
The PM, who was earlier briefed by senior intelligence chiefs in Downing Street, added there was “no alternative conclusion other than that the Russian state was culpable” for the attempted murder of Mr Skripal and his daughter.
She said it was “tragic” that Russian President Putin had “chosen to act in this way”.
The Russian Embassy said the expulsion of 23 diplomats was “unacceptable, unjustified and short-sighted”.
Russian Foreign Minister Sergei Lavrov accused Britain of “playing politics” and ignoring an international agreement on chemical weapons.
He said Moscow would co-operate if it received a formal request for clarification from the UK under the Chemical Weapons Convention, which sets a 10-day time limit for a response.
The UK is to brief the UN Security Council on the investigation at 19:00 GMT, and earlier met Nato’s North Atlantic Council.
At the meeting, Nato allies expressed “deep concern” at the use of a nerve agent and said it was a “clear breach of international norms and agreements”.