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 insisted it had “no motive” 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.
She will make a statement to the House of Commons after midday’s Prime Minister’s Questions.
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.
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.
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”.
Donald Tusk, president of the European Council, said EU leaders would discuss the poisoning at its meeting next week, adding that the incident showed the need for “transatlantic unity”.
The prime minister has also received the backing of US President Donald Trump, Downing Street said, who agreed in a phone call that Moscow “must provide unambiguous answers as to how this nerve agent came to be used”.
France’s President Emmanuel Macron and German Chancellor Angela Merkel have also condemned the attack and offered support to the UK, as well as Estonia, Latvia and Lithuania – the Baltic states bordering Russia, No 10 said.