MOSCOW (Reuters) - Russia hit back at Britain on Wednesday for expelling a diplomat from its London embassy for spying, calling the move groundless and saying Moscow had been forced to respond in kind. The mutual diplomatic expulsions between Russia and Britain are the first since 2007, when relations fell to a low after Kremlin critic Alexander Litvinenko was killed in London with a rare radioactive isotope. The British side took an unfriendly step the other day, having groundlessly declared one of our colleagues in our embassy in London persona non grata, Russias Foreign Ministry said in a statement. We were forced to take an adequate corresponding measure, the statement on the Foreign Ministrys website said. It did not say what the measure was, but London has said Russia requested the removal of a British diplomat on December 16 and that both diplomats had now been withdrawn. The new coalition government has attempted to improve relations with Moscow, but recent Cold War-style espionage claims out of London have raised tensions, drawing a Russian accusation of paranoid spymania. The Russian Foreign Ministry has attacked the handling of the case of Katia Zatuliveter, 25, an aide to a Liberal Democrat on the British parliamentary defence committee. The Sunday Times newspaper reported earlier that she was suspected of espionage. Moscow said responsibility for the situation lay with Britain, but did not say if there would be any long-term effects on easing relations between the two countries, leaving the door open for future acts of reconciliation. It was a shame that the action of the British side was undertaken at a moment when one of the first positive tendencies was appearing in our relationship, the statement said on Wednesday. Prime Minister David Cameron accepted an invitation last month to visit Russia next year and his Foreign Secretary William Hague has already been to Moscow. Tension dates back to the murder of ex-KGB agent Litvinenko, who died in 2006. Moscow has refused to extradite a lawmaker whom Britain says is a suspect in the killing.