"On January 15, the Russian Foreign Ministry summoned UK Charge d'Affaires in Moscow Danae Dholakia and issued a strong protest over the information received by Russia's competent authorities that one of the embassy's diplomatic staff is linked to the UK special services," the ministry said in a statement.
Moscow will not tolerate the activities of undeclared UK intelligence officers in Russia.
"It is stated that Russia's uncompromising line on this issue will continue to be implemented in accordance with the interests of our country's national security. A warning was also issued that if London escalates the situation, the Russian side will give a decisive mirror response," the statement read.
Earlier in the day, the Federal Security Service of Russia (FSB) said that it had identified an undeclared employee of the UK special services who was sent to Russia and posing as secretary of the department of the UK Embassy in Moscow. According to a Sputnik correspondent, Dholakia arrived at the Russian Foreign Ministry around 10.00 a.m. (07:00 GMT) earlier in the day and left in 15 minutes, booed by a large crowd of young people who came to express dissatisfaction with the UK's policy.