Britain has announced that a Russian diplomat has been sent home after the Foreign Office requested he be expelled earlier this month.
The Foreign Office said in a statement there was "clear evidence of activities by the Russian intelligence services against U.K. interests."
The Russian Foreign Ministry today dismissed the expulsion as "groundless" and "unfriendly."
Moscow had made a similar request in response. The Foreign Office said a British diplomat returned from Russia this month.
British Foreign Secretary William Hague denied the British diplomat was an intelligence officer, adding, "We remain open to a more productive relationship with Russia, as with any country, on the basis of respect for our laws," the BBC reported.
The BBC also quoted an unnamed government source as saying the expulsions weren't connected to the case of Katia Zatuliveter, a former parliamentary researcher recently accused of spying for Russia. Moscow has accused London of "spy mania" over her ongoing detention.
This month's expulsions are the first since relations plunged to Cold War lows following the radioactive poisoning of former KGB officer Aleksandr Litvinenko in London in 2006. Many in Britain believe the Russian government was behind the killing of the prominent Kremlin critic.
Earlier that year, Moscow accused British diplomats of espionage using a high-tech fake rock.
At the time, activists from a pro-Kremlin youth group harassed the British ambassador in Moscow and the Kremlin forced some offices of the British Council -- the British government's cultural agency -- to close.
But relations have since steadily improved, marked by Hague's visit to Moscow in October, the highest-level such event since the Litvinenko affair.
Viktor Kremenyuk of Moscow's U.S. and Canada Institute says despite Russia's response to the expulsion of its diplomat, it will have no effect on the trend.
"Those kinds of matters were taken seriously during the Cold War, but now it's seen as not unusual," Kremenyuk says. "The British expel someone, we do the same, and then calm down."
Kremenyuk says Britain especially wants relations to continue improving because it doesn't want to be left out as Russia's ties to Germany and other European countries are growing closer.
"Britain believes being simply a loyal American ally is no longer enough, and that it has to show some independence," he says. "The new Conservative-led government wants to demonstrate its difference in that from the previous Labour government."
Last month, British Prime Minister David Cameron said he would visit Moscow next year.
In other spy news, a youth group connected to the pro-Kremlin United Russia Party announced that Anna Chapman -- one of 10 alleged deep-cover agents expelled from the United States earlier this year -- will become one of its leaders.
written by Gregory Feifer, with agency reports