Image copyright EPA/ Yulia Skripal/Facebook Image caption Sergei Skripal, 66, and his daughter Yulia, 33, are in a critical condition in hospital
Theresa May is set to announce a series of measures against Russia after it failed to meet her midnight deadline to explain how a nerve agent was used to poison a former double agent in the UK.
Russia has denied involvement in the attempted murder of Sergei Skripal and his daughter Yulia in Salisbury.
The Kremlin said any steps by the PM would lead to "retaliatory measures".
Mrs May will be briefed by senior intelligence chiefs in Downing Street ahead of giving an update to MPs later.
Measures could involve the expulsion of Russian diplomats, financial sanctions against wealthy Russians with links to the Kremlin, possible curbs on the Russian-funded TV station RT, or boycotting the Fifa World Cup in Russia later this year.
Shadow chancellor John McDonnell said it was "inevitable" that Mrs May would choose to expel Russian diplomats, adding: "To be honest, it's not that effective".
He told BBC Radio 4's Today programme: "We have to hit them hard where we can, and that's in the pocket".
Former GCHQ director Robert Hannigan said a response should involve "hitting the economic targets" - particularly Russian individuals with assets in London.
He told Today that UK needed to show Russia "the consequences of being a rogue nation".
Downing Street said the prime minister has received the backing of US President Donald Trump, who agreed in a phone call that Moscow "must provide unambiguous answers as to how this nerve agent came to be used".
A No 10 spokesman said France's President Emmanuel Macron and German Chancellor Angela Merkel have condemned the attack and offered support to the UK, as well as Estonia, Latvia and Lithuania - the Baltic states bordering Russia. Image copyright Getty Images Image caption Police continued to work on Tuesday evening near to where the Skripals were found collapsed
Foreign Secretary Boris Johnson said if the attack was shown to be a "direct act" by the Russian state it would be a "clear violation of the chemical weapons convention, a breach of international law and a threat to those who abide by the rules-based international order".
The Foreign Office is set to brief a session of the North Atlantic Council - Nato's political decision-making body - on the Skripal incident later.
Both before and after the deadline had passed, Russia's UK embassy posted a series of tweets saying it would not issue a response without being given access to samples of the nerve agent.
It also contended international obligations required a joint investigation take place into the incident.
Another tweet said it had sought an "explanation" from the Foreign Office, amid speculation the UK could mount a cyber-attack, as it "takes a serious view on cyber security breaches".
Moscow has already threatened to expel British media outlets from Russia if the Kremlin-funded TV channel RT is stripped of its licence to broadcast in the UK.
How could the UK retaliate against Russia?
Britain could expel Russian diplomats, as it did after the poisoning of former Russian Federal Security Service operative Alexander Litvinenko in 2006 with radioactive polonium.
But the BBC's diplomatic correspondent James Landale says many argue that this, and the other measures that were taken after that killing, did not go far enough, and therefore the Skripal response is likely to be much more robust.
So what else could the UK do?
Other possible actions could include:
- Freezing financial assets
- Bans on visas
- Boycotting the Fifa World Cup in Russia later this year
- Taking Russian broadcasters such as RT (formerly Russia Today) off the air in the UK
The actions announced on Wednesday are expected to be those that can be taken unilaterally by the UK - anything co-ordinated with other nations is likely to come later.
Read more on how the UK could retaliate
Mr Skripal and his daughter are critically ill in hospital after being found slumped on a park bench in the centre of Salisbury on 4 March.
Mrs May has said a military-grade nerve agent developed by Russia - part of a group of nerve agents known as Novichok - was used on the pair and it was therefore "highly likely" Russia was involved in the attack.
Det Sgt Nick Bailey, who fell ill responding to the incident, is in a serious but stable condition, and is thought to be improving.
Thirty-five other people had been seen in hospital, of whom 34 had been assessed and discharged, while the condition of one person is being monitored as an outpatient.
Media captionTheresa May: Spy poisoned by "military-grade nerve agent"
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Police are appealing for anyone who saw the Skripals in their Red BMW car - registration plate HD09 WAO - between 13:00 and 13.45 GMT on the day of the poisoning.
The car was left in Sainsbury's upper level car park in the Maltings shopping area before the pair went to the Bishops Mill Pub and then the restaurant Zizzi.
Police confirmed that Mr Skripal, who came to the UK in 2010 as part of a "spy swap" after he had been convicted by Russia of passing information to MI6, was a British citizen.
Meanwhile, it was revealed that UK counter-terror police are leading an investigation into the "unexplained" death of another Russian exile, believed to be Nikolai Glushkov.
Officers say there is no evidence linking the death to the Salisbury attack.
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