Moscow has told Britain it must cut "just over 50" more of its diplomatic and technical staff in Russia in a worsening stand-off over the poisoning of a Russian former spy and his daughter in England, the Russian Foreign Ministry has said.

Key points: Russia has already expelled 23 British diplomats but is ordering "a little over 50" more to be cut

Russia has already expelled 23 British diplomats but is ordering "a little over 50" more to be cut Britain has been given one month to cut its diplomatic contingent in Russia to the same size as the Russian mission in Britain

Britain has been given one month to cut its diplomatic contingent in Russia to the same size as the Russian mission in Britain Russia has also expelled 59 diplomats from 23 other countries — including Australia — for backing Britain

Russia has already retaliated in kind against Britain and ejected 23 British diplomats after frosty relations between London and Moscow deteriorated further over the poisoning of Sergei Skripal and his daughter Yulia.

A request by the Russian Embassy to visit Ms Skripal, whose condition has improved in recent days, is currently being considered by the UK Government.

The Foreign Office said in a statement it was reviewing the Russian request "in line with our obligations under international and domestic law."

Russian officials have insisted they have a legal right to see Ms Skripal.

London says Moscow was responsible for the attack, the first known use of a military-grade nerve agent on European soil since World War Two.

Russia flatly denies that and has cast the allegations as part of an elaborate Western plot to sabotage East-West relations and isolate Moscow.

The Russian Foreign Ministry summoned British Ambassador Laurie Bristow on Friday (local time) and told him London had one month to cut its diplomatic contingent in Russia to the same size as the Russian mission in Britain.

It also expelled 59 diplomats from 23 other countries for backing Britain.

Who is taking action? The United States will expel 60 diplomats and close a Russian consulate in Seattle

will expel 60 diplomats and close a Russian consulate in Seattle Australia has directed two "undeclared intelligence officers" to leave within seven days

has directed two "undeclared intelligence officers" to leave within seven days Britain had earlier booted out 23 Russian diplomats

had earlier booted out 23 Russian diplomats Canada said it was kicking out four diplomats and denying applications to three

said it was kicking out four diplomats and denying applications to three Ukraine , a non-EU country with its own conflicts with Moscow, will expel 13 diplomats

, a non-EU country with its own conflicts with Moscow, will expel 13 diplomats Germany , Poland and France each plan to boot four diplomats

, and each plan to boot four diplomats Lithuania and the Czech Republic will expel three

and the will expel three Italy , the Netherlands , Denmark and Albania will expel two each

, , and will expel two each Sweden, Croatia, Romania, Finland, Latvia and Estonia will expel one each

A spokeswoman for the British Foreign Office called the Russian move regrettable.

She did not say how many diplomatic staff in Russia would be affected. The British Embassy in Moscow says it does not make staff numbers public.

Russian Foreign Ministry Spokeswoman Maria Zakharova said the demand meant Britain would have to cut "a little over 50" more of its diplomatic and technical staff in Russia on top of the 23 diplomats who have already gone home.

"We asked for parity. The Brits have 50 diplomats more than the Russians," Ms Zakharova said.

When asked if that meant London would now have to cut exactly 50 diplomatic and technical staff, she said: "A little over 50."

Britain must now decide how it wants to make the cuts and may be forced to lay off some Russian support staff as well as sending home fully fledged diplomats to satisfy Moscow's demand.

Russia claims British officials searched Aeroflot plane

Russian officials have accused the UK Government of a "blatant provocation". ( Reuters: Regis Duvignau (file photo) )

As relations continued to deteriorate, Moscow has accused the UK Government of "another blatant provocation" after a plane was allegedly searched at London's Heathrow airport.

The embassy claimed a flight run by Russian airline Aeroflot was searched by Border Force officers on Friday (local time) in connection with the diplomatic crisis over the Salisbury spy poisoning.

The Metropolitan Police, which is coordinating the investigation into the Skripals' poisoning denied it was part of the search.

Britain said on Saturday it was routine for border officials to search aircraft.

"It is routine for Border Force to search aircraft to protect the UK from organised crime and from those who attempt to bring harmful substances like drugs or firearms into the country," Security Minister Ben Wallace said in a statement.

Russia's allegations are just the latest in the worsening stand-off.

Britain has an embassy and an ambassador's residence in Moscow and a consulate in the city of Yekaterinburg.

Russia ordered the closure of another British consulate, in St Petersburg, earlier this month in its first retaliatory step.

The Russian measure is similar to one Moscow used against the United States last summer to retaliate against new US sanctions when it ordered Washington to slash its diplomatic staff in Russia by nearly two thirds.

Many local Russian staffers were laid off as a result.

The March 4 poisoning in the southern English city of Salisbury has united much of the West in taking action against what it regards as the hostile policies of President Vladimir Putin.

This includes the United States under President Donald Trump, who Mr Putin had hoped would improve ties.

Sorry, this video has expired Russia, White House react to diplomat expulsion

Reuters