May said that 23 Russian diplomats “identified as undeclared intelligence officers” had one week to leave Britain in a statement to the U.K. parliament on Wednesday.

Russian officials have vowed to expel dozens of British diplomats in retaliation for Prime Minister Theresa May’s expulsion of 23 Russian diplomatic workers Wednesday after a nerve agent attack on a former Russian double agent.

A Russian Foreign Ministry source told the Kommersant business daily that Moscow would reciprocate by expelling British diplomats from the country.

“Our response will be biting. The reaction will be such that Great Britain will regret its actions, Russia’s former ambassador to the United States Sergei Kislyak told the state-run RIA Novosti news agency.

Before May’s speech on Wednesday afternoon, Russia’s Ambassador to Britain Alexander Yakovenko warned London of reciprocal measures for the expulsion of Russian diplomats, Reuters reported.

Russia’s Foreign Ministry has vowed to respond with a statement of its own “in the near future,” the state-run TASS news agency cited ministry spokeswoman Maria Zakharova as saying.

Russia’s Embassy in London labeled the expulsion of its 23 diplomats “totally unacceptable, unjustified and shortsighted.”