Theresa May is expected to announce a series of measures against Russia after it failed to meet her deadline to explain how a nerve agent was used to poison a former double agent in the UK.

Russia has denied any involvement in the attempted murder of Sergei Skripal and his daughter Yulia in Salisbury.

The prime minister is meeting senior intelligence chiefs in Downing Street later.

Mrs May is then expected to give an update to MPs.

Moscow said it would not respond to the UK’s ultimatum until it was given access to the chemical substance used in the attack.

Downing Street says 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 spokesman also said France’s President Emmanuel Macron and German Chancellor Angela Merkel, as well as Estonia, Latvia and Lithuania – Baltic states bordering Russia – have all condemned the attack and offered support to the UK.

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• The spy at centre of poison mystery
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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.

The UK government has not publicly disclosed the measures it is considering against Moscow.

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

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 4 March and it was therefore “highly likely” Russia was involved in the attack.

On Tuesday, police said Mr Skripal and his daughter remained critically ill in hospital after being found slumped on a park bench in the centre of Salisbury.

Det Sgt Nick Bailey, who fell ill responding to the incident, is in a serious but stable condition, and is thought to be improving.

Assistant Commissioner Neil Basu said 35 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.

He appealed for witnesses who saw the Sergei and Yulia Skripal 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.

• Up to 14 other deaths to be re-examined for Russian links
• No change to England World Cup plans

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.

Elsewhere on Tuesday, 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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