British Prime Minister Theresa May expelled 23 Russian diplomats on Wednesday, blaming Moscow for the poisoning of a former Russian spy and sharply escalating the dispute between the two countries.

Her statement to Parliament came after Moscow rejected a British deadline for Russia to explain itself over this month’s nerve agent attack on the former spy, Sergei V. Skripal, and his daughter. The two countries have engaged in an worsening clash in recent days, with Britain widening an investigation into the incident and courting friends and allies to increase pressure on Russia, while Moscow has threatened to retaliate over any punitive action.

“This represents an unlawful use of force by the Russian state against the United Kingdom,” Mrs. May said in an address to the House of Commons. “It must therefore be met with a full and robust response.”

She said she had agreed with Britain’s National Security Council to suspend all high-level contacts between her country and Russia, and to expel 23 Russian diplomats, who had one week to leave. She described it as the biggest expulsion in more than 30 years.

“Their response has demonstrated complete disdain for the gravity of these events,” Mrs. May said of Russia’s response. “They have provided no credible explanation.”

The Russian foreign minister, Sergey V. Lavrov, said on Wednesday that Britain was “acting out political drama” rather than investigating the matter seriously.

“Russia could not have had any motives” for the attack, he said, “but those who would like to continue a Russophobic campaign in absolutely all areas of human activities” could have had such motives.

 Russia’s Ambassador to Britain Alexander Yakovenko warned London on Wednesday of reciprocal measures if Britain expelled Russian diplomats, the RIA news agency said.