The UK will be expected to cut around 50 personnel from its diplomatic missions in Russia as tensions between the two nations deteriorate further following the poisoning of a former Russian spy and his daughter in England.
Russian Foreign Ministry spokeswoman Maria Zakharova told CNN on Saturday that the number was calculated to ensure the two nations achieve parity in how many staff members they have working at their diplomatic missions.
Moscow initially expelled 23 British diplomats after 23 Russian diplomats were told to leave the UK.
Russia is now insisting that more leave so the staff will be the same size in the countries' respective embassies.
More than 25 countries announced this week that they would expel Russian diplomats in support of Britain, which blames Moscow for the March 4 nerve agent attack against Sergei Skripal and his daughter, Yulia, in the southern English city of Salisbury. Russia denies involvement.
Also Saturday, Russia's Ministry of Transport said it planned to ask the UK why authorities searched a Moscow-bound Aeroflot plane Friday at London's Heathrow Airport.
On its website, the ministry said, "In the absence of an explanation, the Russian side will regard these actions against our aircraft as illegal, and also reserves the possibility of similar actions against British air carriers."
The UK Security Minister said the search was "routine."
"It is routine for Border Force to search aircraft to protect the UK from (organized) crime and from those who attempt to bring harmful substances like drugs or firearms into the country," spokesman Ben Wallace said in a statement to CNN. "Once these checks were carried out the plane was allowed to carry on with its onward journey."
"We are considering requests for consular access in line with our obligations under international and domestic law, including the rights and wishes of Yulia Skripal," a spokeswoman for the UK's Foreign and Commonwealth Office said Saturday.
On Friday, the Russian Embassy in London tweeted, "Good news as Yulia Skripal is reported as recovering well. We insist on the right to see her, in accordance with the 1968 Consular Convention."