The head of Russia's SVR foreign intelligence agency said on Wednesday the poisoning of a Russian former double agent in Britain was a "grotesque provocation" by the British and US security services.
The Russian foreign ministry said in a statement that it had summoned the heads of missions from 23 countries earlier to tell them that some of their diplomats had to leave the country.
A second police officer is being treated in hospital for suspected poisoning after a chemical attack on a former Russian double agent but the symptoms are minor, the Mail Online reported on Thursday.
British Prime Minister Theresa May visited Salisbury on Thursday, the elegant cathedral city which became the unlikely backdrop to a chemical attack against a Russian former double agent this month.
Britain's expulsion of 23 Russian diplomats following the poisoning of a former double agent is a hostile and unjustified action, Russia's embassy in London said on Wednesday.
"Moscow does not accept unfounded accusations that are not based on evidence and a language of ultimatums," Kremlin spokesman Dmitry Peskov told reporters.