SOFIA (Reuters) - The Russian government has expelled a Bulgarian diplomat in a tit-for-tat move after Sofia asked Moscow to recall a diplomat on suspicion of espionage, Bulgaria’s foreign ministry said on Thursday.

Sofia’s ambassador in Moscow, Atanas Krastin, has been informed that the diplomat must leave Russia within the next 24 hours in line with diplomatic protocol, the ministry said in a statement. Moscow cited the “principle of reciprocity”, it said.

“This is the mirror image response we warned about,” the RIA news agency cited a source in Russia’s foreign ministry as saying on Thursday.

Bulgaria, a member of NATO and the European Union but with close historic ties to Moscow, said in October it had requested the Russian diplomat’s recall after Bulgarian prosecutors determined that he had been involved in espionage.

The prosecutors said the expelled diplomat had held conspiratorial meetings since September last year with Bulgarians, including a senior official who had clearance for classified NATO and EU information.

Bulgaria also declined to grant a visa to Russia’s incoming defense attache. It said at the time that one of the bodies involved in agreeing long-term visas for diplomats had come to a “negative opinion” about Russia’s putative defense attache.

Russia strongly opposed the expansion of NATO and the EU into former communist eastern Europe. Bulgaria was Moscow’s most reliable ally in the region during Soviet times.

Despite periodic strains in their post-Soviet ties, however, Russia remains the biggest supplier of energy to Bulgaria, which also declined to join its NATO and EU allies last year in expelling Russian diplomats over the poisoning of a former Russian spy, Sergei Skripal, in Britain.