British Prime Minister Theresa May says 18 countries have announced they are expelling more than 100 Russian intelligence officers in response to the nerve-agent poisoning of a former spy.

The U.S., Canada, Ukraine and 15 European nations have joined Britain in ordering out Russian diplomats who are accused of being spies working under diplomatic cover.

May told lawmakers in the House of Commons that the action is the "largest collective expulsion of Russian intelligence officers in history."

The coordinated expulsions are a victory for U.K. attempts to rally an international response to the use of a nerve agent against Sergei Skripal and daughter Yulia in the English city of Salisbury.

May says that Russia's "increasingly aggressive" behavior is a threat to the West's collective security.