LONDON - British Prime Minister Theresa May announced measures Wednesday against Russia over its failure to respond to a demand by the British government to explain how a Russian-made nerve agent was used in a recent attack in Salisbury.

Former spy Sergei Skripal, 66, and his daughter Yulia, 33, are in a critical condition in hospital after being exposed to a nerve agent earlier this month.

The Russian government has denied any involvement in the attack, and has threatened reprisals against Britain if measures are taken by the British government.

May announced that 23 Russian diplomats will be expelled from Britain and told to leave within a week. She said it is the biggest expulsion from Britain in 30 years.

The prime minister said Britain will not break off diplomatic relations with Russia, but all high level bilateral contacts will be cancelled.

She said no government ministers or members of the Royal family will attend the World Cup being hosted by Russia.