Russia's expulsion of 23 British diplomats “doesn't change the facts of the matter” of the poisoning of a former double agent in an English city, Prime Minister Theresa May said on Saturday.

Russia was “in flagrant breach of international law,” she told her Conservative Party's spring forum, adding that Britain “will consider our next steps in the coming days”.

“Russia's response doesn't change the facts of the matter ─ the attempted assassination of two people on British soil, for which there is no alternative conclusion other than that the Russian State was culpable,” she said.

May blames Russia for the nerve agent attack on former spy Sergei Skripal and his daughter Yulia in the English city of Salisbury on March 4, which has left them both fighting for their lives.

She warned that Britain “will never tolerate a threat to the life of British citizens and others on British soil from the Russian government.” But she said Britain had “no disagreement with the Russian people”.

Earlier this week, Britain announced the expulsion of 23 Russian diplomats and suspension of high-level contacts over the poisoning.

Moscow responded on Saturday by expelling 23 British diplomats from Moscow in a tit-for-tat measure. It also said it would halt the activities of the British Council, the country's international organisation for cultural relations, in Russia.

“We are profoundly disappointed at this development,” the British Council said in a statement.

“It is our view that when political or diplomatic relations become difficult, cultural relations and educational opportunities are vital to maintain on-going dialogue between people and institutions.”