Russia's Foreign Ministry has summoned the British ambassador to Russian Federation for talks in a heightening dispute over a nerve agent attack on a former spy in Britain.
Britain's National Security Council will meet early next week to consider the next steps, May said.
They said there "no plausible alternative explanation" for the use of the Soviet-designed nerve agent other than Russian responsibility.
Relations between the two nations have deteriorated rapidly since the March 4 nerve agent attack on Sergei Skripal, 66, and his daughter Yulia, 33, in the English city of Salisbury. She said Britain is taking a tough line because of frustration at recent advances of Russian-backed Syrian government forces against Western-backed rebels.
It said this was a response to Britain's "provocative actions" and "baseless accusations over the incident in Salisbury on March 4".
Bristow told reporters afterwards that Britain had only expelled the Russian diplomats after Moscow had failed to explain how the nerve toxin had got to Salisbury.
Britain said on Saturday its priority was looking after its staff in Russian Federation, after Moscow expelled 23 British diplomats in a escalating crisis between the two countries over the poisoning of a double agent.
The announcement came after the British ambassador to Russia, Laurie Bristow, was summoned to the foreign ministry in Moscow.
Russia has now denied that any program under the name Novichok ever existed, despite the evidence presented two decades ago by the Russian scientist Vil Mirzayanov, who revealed its existence after becoming concerned it violated Russia's commitments to the Chemical Weapons Convention.
The Kremlin has indicated it would expel British diplomats in a riposte to London's move as well as adopt other measures that would "most suit Moscow's interests".More news: Fourth-seeded Texas A&M eliminates Drake
Following the meeting, Mr Bristow said the United Kingdom had no quarrel with the Russian people and would "always do what is necessary to defend ourselves".
TRT World's Dana Lewis reports.
British police said there is no apparent link between the attack on Glushkov and the poisoning of the Skripals, but both have raised alarm in the West at a time when Russian Federation is increasingly assertive on the global stage and is facing investigations over alleged interference in the Donald Trump's 2016 election as USA president.
Britain has escalated a war of words with Russian Federation over the incident in recent days.
On Friday, Foreign Secretary Boris Johnson said it was "overwhelmingly likely" that Russian President Vladimir Putin had ordered the nerve agent attack.
Britain, the United States, Germany and France have jointly called on Russia to explain the attack, while US President Donald Trump has said it looks as if the Russians were behind it. This use of a military-grade nerve agent, of a type developed by Russian Federation, constitutes the first offensive use of a nerve agent in Europe since the Second World War.
The Skripals remain in a critical condition following the attack that also poisoned a British police officer.
Elsewhere, the Met Police have launched a murder investigation into the death of businessman and Kremlin critic Nikolai Glushkov - a Russian exile who was found dead in his south-London home on Monday.
Russian Federation offered some cooperation to British authorities after the 2006 London murder of ex-KGB agent Alexander Litvinenko too.
Britain said the assistance, in that case, was not enough, and in 2016, a judge-led inquiry concluded that Putin had probably approved Litvinenko's murder, something Moscow denies.