France: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.
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.