Moscow initially expelled 23 British diplomats after 23 Russian diplomats were ordered out by London.

But now the Russians are insisting that even more UK diplomats should leave.

It says the UK and Russian diplomatic missions should be the same size in each country.

In practice that would mean at least another 27 UK officials being sent home, the BBC's Paul Adams reports from Moscow.

The British government blames Russia for the nerve agent attack on Sergei and Yulia Skripal. Moscow denies it.

On Friday the British ambassador in Moscow, Laurie Bristow, was told that the UK had a month to cut its diplomatic mission in Russia to the same size as the Russian mission in Britain.

On Saturday, Russian foreign ministry spokeswoman Maria Zakharova told Reuters news agency that that meant Britain would have to cut "a little over 50" of its diplomats in Russia.

"We asked for parity," she said.

A UK Foreign Office spokeswoman said: "The Russian response is regrettable but in light of Russia's previous behaviour, we anticipated a response.

"However, this doesn't change the facts of the matter: the attempted assassination of two people on British soil, for which there is no alternative conclusion other than that the Russian state was culpable.

"Russia is in flagrant breach of international law and the Chemical Weapons Convention and actions by countries around the world have demonstrated the depth of international concern."

Some 150 Russian diplomats have been expelled by mainly Western countries as their governments have rallied behind Britain.

Russia initially hit back at the UK, but then announced 60 US expulsions. On Friday it called in a string of foreign ambassadors with news that their own countries' measures were being matched.

Sergei and Yulia Skripal collapsed in Salisbury, southern England, on 4 March. Mr Skripal remains in a critical but stable condition. Yulia's condition is said to be improving.

UN Secretary General Antonio Guterres on Thursday issued a warning over "a situation that is similar, to a large extent, to what we lived during the Cold War".

Meanwhile, in a separate development, Russia's ministry of transport has demanded an official explanation over the alleged searching of an Aeroflot passenger plane by UK officials at London's Heathrow airport on Friday.

"If there is no explanation, the Russian side will deem the actions towards our plane as illegal and also reserve the right to take similar action against British airlines," it said in a statement.

The UK has yet to confirm any such search took place.

BBC