Russia's ambassador to the EU has suggested the nerve agent that poisoned an ex-spy and his daughter in a sleepy English city may have come from a lab in the UK.
"When you have a nerve agent or whatever, you check it against certain samples that you retain in your laboratories. And Porton Down, as we now all know, is the largest military facility in the United Kingdom that has been dealing with chemical weapons research. And it's actually only eight miles from Salisbury," Vladimir Chizhov told the BBC.
The UK's foreign secretary has dismissed the suggestion.
"We'll be welcoming some technical experts from the Organisation for the Prohibition of Chemical Weapons in The Hague and they will be coming to the UK. We'll be sharing the samples with them, as you would expect. They have a lot of expertise. They will then be testing them in international... internationally reputable laboratories," Boris Johnson told reporters.
The UK has accused the Kremlin of orchestrating a nerve toxin attack on a former Russian double agent and his daughter in the sleepy English city of Salisbury.
Sergei Skripal and his daughter Yiulia have been fighting for their lives in hospital since they were found unconscious on a bench on March 4.
Relations between London and Moscow have crashed to a post-Cold War low over the Salisbury attack. It is the first known offensive use of a nerve agent in Europe since the Second World War.
Russia has complained that Britain has failed to provide any evidence of its involvement in the Salisbury attack. Moscow says it is "shocked and bemused" by the allegations.
On Saturday, Russia formally expelled 23 British diplomats in a retaliatory move against London. The Russian Foreign Ministry says they have one week to leave the country.
The Russian Foreign Ministry summoned the British ambassador, Laurie Bristow, to its headquarters on Saturday morning to inform him of the retaliatory measures.
Russia also says it is shutting down the activities of the British Council, which fosters cultural links between the two countries. The UK's consulate-general in St Petersburg is also being shut.
Russia's response is more robust than expected. The closure of the British Council's Moscow office will sever cultural ties. The closure of the consulate in St Petersburg will end the UK's diplomatic presence in Russia's second city.
How did the UK respond to this?
Robustly itself. Bristow told reporters afterwards that Britain had only expelled the Russian diplomats after Moscow had failed to explain how the nerve toxin had got to Salisbury.
The UK Foreign Ministry said it had anticipated Russia's response and its priority is to look after its staff in Russia and assist those returning home.
"Russia's response does not 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," the ministry said in a statement.
The UK's National Security Council is due to meet early in the week to consider London's next steps.
Why is Russia doing this?
It follows the UK's decision last Wednesday to expel 23 Russian diplomats over the attack. It left former spy Sergei Skripal and his daughter Yiulia fighting for their lives in hospital.
A British police officer was also poisoned when he went to help them and remains in a serious but stable condition.
Moscow announced the measures on the eve of Russia's presidential election which incumbent Vladimir Putin is expected to win comfortably.
The Foreign Ministry said Moscow's measures were a response to what it called Britain's "provocative actions and unsubstantiated accusations". It warned London that it stands ready to take further measures in the event of more "unfriendly steps."
An escalating war-of-words
On Friday, the UK Foreign Secretary Boris Johnson said it was "overwhelmingly likely" that Russian President Vladimir Putin himself took the decision to use a military-grade nerve toxin to strike down a former Russian agent in the quiet english cathedral city of Salisury.
On Wednesday, Prime Minister Theresa May said the Russian state was "culpable" for the attempted murder of Sergei Skripal, a former double agent and his daughter, Yiulia. May said it was "tragic" that Putin, who is likely to win a fourth term in office in Russia's presidential election on Sunday, had chosen to act in such a way.
A british police officer is also being treated in hospital for contamination.
May said the UK would consider its next steps with its allies in the coming days.
"We will never tolerate a threat to the life of British citizens and others on British soil from the Russian government. We can be reassured by the strong support we have received from our friends and allies around the world," May said.
The UK, the US, Germany and France have jointly called on Russia to explain the attack. US President Donald Trump has said "it looks as if" the Russians were behind it.
What did Boris say?
"We have nothing against the Russians themselves. There is to be no 'Russophobia' as a result of what is happening," Johnson told reporters at the Battle of Britain bunker from which World War Two fighter operations were controlled.
"Our quarrel is with Putin's Kremlin and with his decision and 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."
What has Russia said?
The TASS news agency quoted the Kremlin as saying accusations that President Putin was involved in the nerve agent attack were shocking.
"Any reference or mention of our president in this regard is a shocking and unforgivable breach of diplomatic rules of decent behaviour," Kremlin spokesman Dmitry Pesko said, according to TASS.
Russia has repeatedly denied any involvement in the Skripal case.
Moscow says it is open to cooperation with the UK. However, it has refused British demands to explain how Novichok, a nerve agent developed by the Soviet military, was used against the Skripals.
Russian Foreign Ministry spokeswoman Maria Zakharova told reporters the most likely source of the toxin was the UK itself or the Czech Republic, Slovakia, Sweden or the United States.
She said these countries have been testing the substance since the end of the 1990s.
This assertion has not been verified. However, The Swedish, Czech and Slovak foreign ministers and ministry have all rejected the claim separately.
On Friday, Russian investigators said they had opened a criminal investigation into the attempted murder of Yiulia Skripal. They offered to cooperate with the British authorities.
What are relations like between the UK and Russia now, then?
Not good. The Kremlin has cast Britain as a post-colonial power unsetlled by the plans for Brexit. It has even suggested London fabricated the attack in an attempt to whip up anti-Russian hysteria.
Links have been strained since the murder of the ex-KGB agent Alexander Litvinenko with radioactive polonium-210 in London in 2006. A British inquiry said his death was probably approved by Putin.
The Kremlin has repeatedly denied any involvement in Livinenko's killing.