Moscow has vowed there will be a swift response to the expulsion of 23 of its diplomats by Britain.
President Vladimir Putin would make a final decision and “won’t take long” to respond, his spokesman said.
The UK took its action after Moscow refused to explain how a Russian-made nerve agent was used on a former spy and his daughter in the UK.
Russia denies it was involved. Its foreign ministry called the allegations by Prime Minister Theresa May “insane”.
Foreign Minister Sergei Lavrov has been speaking on the issue at an event in Moscow, although Russian media reports of what he has said have been conflicting.
The RIA Novosti agency quoted him as saying Russia would “definitely” expel UK diplomats “soon”.
But he was quoted by others as saying that Russia would inform British authorities first before any media announcement. “This is, in my view, being polite; this is how gentlemen behave,” he said.
Mr Lavrov was also quoted by Tass as saying he hoped the former spy, Sergei Skripal, would recover so he could shed light on events.
Interfax quoted the foreign minister as saying a motive behind the UK’s actions might be to complicate Russia’s hosting of Fifa’s World Cup this summer.
President Putin’s spokesman, Dmitry Peskov, said the foreign ministry was working out measures but added that “it is indeed the president who will make the final decision”.
Foreign ministry spokeswoman Maria Zakharova condemned what she said were “absolutely insane accusations made by the UK prime minister against Russia”, adding that the UK had failed to provide any factual information about the case.
She said diplomatic notes sent to the Foreign Office had received replies that “made no sense”.
Foreign Secretary Boris Johnson stepped up the war of words with Russia on Thursday.
He said the attack was Russia’s “way of saying to people this is what happens to people who stand up to our regime”.
Mr Johnson said: “Now is the moment for Putin to jam the lid down and send a signal to people: ‘You do this, you’re going to die’.”