British Ambassador Laurie Bristow was summoned by Russian foreign ministry again on Friday [Reuters]

The Kremlin 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 - an allegation Russia denies.

On Saturday, Foreign Ministry Spokeswoman Maria Zakharova told the media that meant Britain would have to cut "a little over 50" of its diplomats in Russia saying Russia is asking for parity as the Brits have 50 diplomats more than the Russians.

Over the past few days, the UK and 29 other countries including the US have expelled more than 150 Russian diplomats. In return, Russia decided to expel 150 Western diplomats, of whom 23 were British diplomats and 60 were Americans, and asked for the closures of the US Consulate in Saint Petersburg.