Russia is set to expel British diplomats after 23 Russian officials were ordered to leave the UK, as relations between the countries crashed to a post-Cold War low over the Salisbury nerve agent attack.
In what is believed to be the first use of a nerve agent in Europe since the Second World War, former Russian spy Sergei Skripal, 66, and his daughter Yulia Skripal, 33, were poisoned in Salisbury on Sunday 11 March.
European leaders have joined the UK in condemning the attack, which they have said could only have been carried out by Russia.
British Prime Minister Theresa May has ordered 23 Russian diplomats she said were undeclared intelligence officers to leave the country within a week, along with several other measures.
RIA news agency quoted Russia's ambassador to Britain as saying the diplomats are due to leave London on 20 March.
Russian Foreign Minister Sergei Lavrov told Reuters "we will, of course" expel British diplomats in retaliation.
Russia has refused Britain's demands to explain how the nerve agent Novichok, developed by the Soviet military, was used to strike down Mr Skripal and his daughter.
British Foreign Minister Boris Johnson yesterday also stepped up the rhetoric against Russia's government, directly linking the attack to Russian President Vladimir Putin.
Speaking during a visit to a west London military bunker with the Polish foreign minister, Mr Johnson said the UK's "quarrel is with Putin's Kremlin".
"We think it overwhelmingly likely that it was his decision to direct the use of a nerve agent on the streets of the UK, on the streets of Europe, for the first time since the Second World War," he said.
Russia denies involvement and said the accusations against Mr Putin were "shocking and unforgivable".
UK Home Secretary Amber Rudd also announced that a string of deaths on UK soil are to be reinvestigated by the police and MI5 after claims of Russian involvement.
On Friday, Nato Secretary General Jens Stoltenberg said Nato had "no reason to doubt the findings and assessments by the British government".
He said the "UK is not alone" and Nato allies gave "strong political support" to Britain.
Mr Stoltenberg said the incident was part of a "pattern of reckless behaviour" from Russia, following allegations of cyber attacks and election meddling in recent years.
"It is important that Russia gets a clear signal that it costs to behave the way they behave," Mr Stoltenberg said.
"I'm absolutely certain that Russia has underestimated the resolve and unity of Nato allies when we have implemented different kinds of sanctions over the last years," Mr Stoltenberg added.
But Kremlin spokesman Dmitry Peskov said Russia was not worried by international expressions of support for the UK and challenged Britain to "provide some confirmation".
He said: "Sooner or later, the British will have to show some proof to those 'colleagues' who say they are with UK on this; sooner or later will have to stand up its accusations."
Ahead of an EU leaders' summit, German Chancellor Angela Merkel said discussions of a potential boycott of the World Cup in Russia are "not an immediate priority".
No symptoms for others potentially exposed
Wiltshire Police said 131 people had been identified as potentially being exposed to the nerve agent - but none has shown any symptoms.
Salisbury District Hospital has also assessed 46 people who came forward expressing health concerns but they were not admitted.
In a letter to the Times, Salisbury NHS Trust emergency medical consultant Stephen Davies said only three people - the Skripals and Det Sgt Nick Bailey - had needed treatment.
Det Sgt Bailey remains in a serious but stable condition in hospital after being contaminated with the chemical.
Russia's ambassador to Britain Alexander Yakovenko claimed the UK had angled allegations against Russia to "divert attention from Brexit".
He criticised the lack of transparency and said: "Nobody saw even the pictures of these people in a hospital, whether they are alive or maybe they are in good health. Nobody talked to the doctors."
Russia's Foreign Minister Sergei Lavrov said Russia had asked the UK to take action under the Chemical Weapons Convention.
He also responded to Defence Secretary Gavin Williamson - who previously said Russia should "go away" and "shut up" - saying: "Maybe he lacks education."
Some 220 police officers from 16 forces, 80 ambulance staff, 50 fire officers, 200 armed forces personnel and 250 specialist officers have so far been deployed as part of the investigation, Wiltshire Police said.
- BBC / Reuters