Russia said on Thursday it was expelling the British defence attache and promised further, unspecified retaliation after Britain threw out his Russian counterpart last week.
Russia's Foreign Ministry said it had summoned a British embassy representative to inform them of the move.
On May 8, Britain announced it was ordering out the Russian defence attache, describing him as an undeclared military intelligence officer.
The Russian statement said Moscow viewed that as "a politically motivated action with a clearly expressed Russophobic character, which deals irreparable damage to bilateral relations".
The British diplomat, Adrian Coghill, was given a week to leave the country. Moscow said that would not be the end of the matter and Britain would be informed about further retaliatory steps.
Relations between the two countries were in dire shape even before the Ukraine war and have worsened further since it started. Britain has played a prominent role in mobilising support for Ukraine and providing it with weapons.
Britain said last week it was also removing diplomatic status from some Russian-linked promises and imposing new visa restrictions to cap the length of time that Russian diplomats can spend in Britain.
