The UK expelled 23 Russian diplomats in the wake of the poisoning of Sergei Skripal, 66, and his daughter Yulia with the nerve agent Novichok.

A number of British allies have also ejected Russian diplomats in a show of solidarity following the attack.

So far, more than 150 Russian diplomats, some of whom are suspected spies, have been ordered to return home by countries including the US, Germany, France and Canada.

Russia has ordered Britain to reduce the number of diplomats in Moscow. Pic: AFP/Getty Images

The tit-for-tat diplomatic expulsions were launched by Russia on Thursday.

Russian foreign minister Sergei Lavrov said Moscow would retaliate by expelling the same number of diplomats that each country had ejected.

The ministry said it summoned the British ambassador on Friday following the “provocative and unsubstantiated actions by Britain, which instigated the expulsion of Russian diplomats from various nations for no reason”.

Daughter of ex-Russian spy Sergei Skripal '˜conscious and talking'

Ex-spy Sergei Skripal is still in a critical condition.

The British Government was given 30 days to reduce its diplomatic personnel to the same number of diplomats that Russia has stationed in the UK.

The number of British diplomats that will have to leave Russia was not immediately clear.

A spokeswoman for the Foreign and Commonwealth Office said: “It’s 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.

Yulia Skripal is now "improving rapidly".

“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.”

She said the FCO would not be commenting on the details of the meeting.

On Thursday, it emerged that Ms Skripal, 33, is now “improving rapidly” after she was left critically ill when she was exposed to Novichok on March 4.

The BBC reported separate sources have verified Yulia is both conscious and talking.

Her father remains in a critical but stable condition in hospital.

Scotland Yard believes Mr Skripal and his daughter, who was visiting him from Russia, first came into contact with the deadly chemical at his home.

Detailed forensic testing revealed the highest concentration of Novichok was found on the front door.

The vast attempted murder investigation continues to unfold in the Wiltshire city, most recently leading police to cordon off a children’s play area at Montgomery Gardens.

Detectives said traces of the nerve agent had been found at some of the other scenes where investigations had been carried out, but at lower concentrations.

Efforts will now focus in and around the address, and specialist teams will step back from some of the other areas investigated over the past few weeks.

Mr Skripal and his daughter have been patients at Salisbury District Hospital since they were discovered unconscious on a park bench close to The Maltings shopping centre nearly four weeks ago.

Wiltshire Police Detective Sergeant Nick Bailey, who was also exposed to the nerve agent, was discharged from the hospital last week.

Russia orders UK to reduce number of diplomats in Moscow