United Kingdom Prime Minister Theresa May on Wednesday ordered 23 Russian diplomats to leave the country in connection with the attack on a former Russian double agent in Salisbury on March 4, BBC reported. May said the “undeclared intelligence officers” had a week to leave the United Kingdom. The announcement comes after Moscow rejected her ultimatum to respond to her allegations that Russia was involved in the attack.

This is the biggest expulsion in three decades, Reuters quoted May as saying. The United Kingdom will freeze any assets owned by Moscow that are perceived as a threat, and minimise its presence at the Russia-hosted World Cup.

May had asked Moscow to explain why a Russian-made nerve agent was used in the attack on former spy Sergei Skripal. The UK prime minister had said she will have to conclude that there was an “unlawful use of force” by Moscow if they did not respond by Tuesday night.

Russia’s Foreign Minister Sergei Lavrov had rejected claims about Russia’s role in the attack. Lavrov said Russia’s requests to see the samples of the nerve agent had been turned down which, he claimed, violated the chemical weapons convention that outlaws the production, stockpiling, and use of chemical weapons. He said under the convention, Moscow has 10 days to reply to an official accusation.

The attack

Skripal and his daughter Yulia were found unconscious on a bench outside the Maltings shopping centre in Salisbury. Both of them, and a police officer who first responded to them, are said to be in serious condition in hospital. Police said the substance that rendered them critically ill was a nerve agent.

UK health officials on Sunday asked hundreds of people who visited a restaurant and a pub where the attack took place to wash their clothes and wipe personal items like handbags and phones, in case the substance came in contact with them.