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 slaying of Sergei Skripal and his daughter Yulia in Salisbury.
The prime minister is satisfying 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 countries bordering Russia – have all condemned the attack and offered support to the UK.
What we know so far What are Novichok nerve agents ? The spy at centre of poison mystery Russia retaliation could hurt UK business Image caption Freezing financial assets Outlaw on visas Boycotting the Fifa World Cup in Russia afterward this year Taking Russian broadcasters such as RT( formerly Russia Today) off the air in the UK Media captionTheresa May: Spy poisoned by “military-grade nerve agent”