The Foreign Ministry 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," the Foreign Ministry said in a statement.

"We were forced to take an adequate corresponding measure," said the statement on the Foreign Ministry's web site.

It did not say what the measure was, but London has said Russia requested the removal of a British diplomat on Dec. 16 and that both diplomats had now been withdrawn.

The dispute undermines efforts by Britain's new coalition government to forge a better relationship with Russia.

Britain has said it requested the expulsion of the diplomat on Dec. 10 after evidence of Russian intelligence service activities against British interests.

"There is clear evidence of activities by Russian intelligence services against U.K. interests," a British Foreign Ministry spokesman said Wednesday.