British Prime Minister Theresa May has announced 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.
Mrs 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.
The Russian Embassy said the expulsion of 23 diplomats was "unacceptable, unjustified and short-sighted".
It is the largest mass expulsion since 31 were ordered out in 1985 after double agent Oleg Gordievsky defected.
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.
Police officer Nick Bailey also fell ill responding to the incident, but was in a serious but stable condition and was 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.
UK's response after Russia refused to meet its deadline:
- 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
- Plans to consider new laws to increase defences against "hostile state activity"
- 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".
She said the use of a Russian-made nerve agent on UK soil amounted to the "unlawful use of force".
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.
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.
Speaking ahead of the PM's statement, a spokesman for Russian president Vladimir Putin said: "Moscow won't accept absolutely unfounded accusations against it, which are not substantiated by any evidence, and won't accept the language of ultimatum."
Mrs May said it was "tragic" that Russian President Vladimir Putin had "chosen to act in this way".
The UK is to brief the UN Security Council on the investigation at 7pm 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".
Mrs May has welcomed support from allies including the US, Nato and the EU, and said Britain would be pushing for a "robust international response" at the UN Security Council later.
"This was not just an act of attempted murder in Salisbury - nor just an act against UK," she said.
"It is an affront to the prohibition on the use of chemical weapons. And it is an affront to the rules-based system on which we and our international partners depend."
Mr Skripal, a British citizen, came to the UK in 2010 as part of a "spy swap" after he had been convicted by Russia of passing information to MI6.
The Foreign Office has updated its advice on travel to Russia, saying "heightened political tensions" mean Britons should "be aware of the possibility of anti-British sentiment or harassment at this time".
In a statement, the FA said it would work closely with the UK government and authorities regarding its participation in the World Cup in June.
Meanwhile, British police and army sealed off areas of the north Dorset town of Gillingham as part of the attempted murder investigation.
A wide cordon is in place around a truck, thought to have recovered Mr Skripal's car from Salisbury.
About 180 troops have been deployed to Salisbury to assist with removing vehicles and objects from affected areas, while the Zizzi restaurant and Bishop's Mill pub where the Skripals visited before falling ill remained closed.
Police from 15 departments across England and Wales have been sent to Wiltshire to support the investigation.
Chemical weapons expert Hamish de Bretton Gordon told the BBC he believed the nerve agent was produced in a military research base in Shikhany in central Russia.
He said it was a secure location and if the Russians were serious about cooperating in any investigation, they should give the Organisation for the Prohibition of Chemical Weapons (OPCW) access to the site.