Russia has already retaliated in kind against Britain and ejected 23 British diplomats over the poisoning of former Russian spy Sergei Skripal and his daughter Yulia. London says Moscow was behind the attack, something Russia denies.
British Ambassador Laurie Bristow was summoned again on Friday and told London had one month to cut its diplomatic contingent in Russia to the same size as the Russian mission in Britain.
On Saturday, Foreign Ministry Spokeswoman Maria Zakharova told Reuters that meant Britain would have to cut "a little over 50" of its diplomats in Russia.
"We asked for parity. The Brits have 50 diplomats more than the Russians," said Zakharova.
When asked if that meant London would have to bring home exactly 50 diplomats, she said: "A little over 50."