Updated March 21, 2018 12:41:25

Nearly two dozen Russian diplomats expelled by Britain over the poisoning of an ex-spy have arrived home, while a scientist involved in the creation of the nerve agent said it could be manufactured by other countries.

Former Russian double agent Sergei Skripal and his daughter Yulia are in critical condition in the English city of Salisbury after being poisoned on March 4.

Britain said they were poisoned with a class of nerve agent known as Novichok, and blamed Russia for the attack.

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Russian scientist Leonid Rink told the state news agency RIA Novosti that Britain and others could easily synthesize Novichok after chemical expert Vil Mirzayanov emigrated to the US and revealed its formula.

"It's easily available to professionals, and there is no problem for Britain, the US and other developed nations to create such a weapon," he said.

Mr Rink said Novichok had a different name when it was designed in the Soviet Union, arguing that British officials used the name Novichok to convince the public that Russia was to blame for the poisoning.

Photo: Adults and children board an aircraft sent to collect Russian diplomats and their families. (Reuters: Chris Radburn)

Britain has dismissed claims the nerve agent could come from the UK.

On Sunday, Russia's ambassador to the EU suggested the nerve agent could have come from Britain's chemical weapons research facility, Porton Down.

But the British Government said that was "nonsense".

Bags in arms, Russians leave UK

Photo: Staff wave goodbye as colleagues and their children board buses outside Russia's Embassy in London. (Reuters: Toby Melville)

On March 14, British Prime Minister Theresa May gave the 23 diplomats — whom she said were undeclared intelligence agents — a week to leave Britain. Russia responded by expelling the same number of British diplomats, who are expected to leave in the coming days.

Diplomats and their families emerged from the Russian Embassy in west London with suitcases, bags and pet carriers. Some hugged before boarding vehicles to Stansted Airport near London for a flight to Moscow that landed later on Tuesday.

Russia has fiercely denied any involvement, saying that it had no motive to kill Mr Skripal, who was convicted of spying for Britain but released in a 2010 spy swap.

Photo: Sergei Skripal and Yulia Skripal remain in a critical condition. (AP: Misha Japaridze, Facebook: Yulia Skripal)

It said that it had completed the destruction of its chemical arsenals last year under international oversight.

White House spokeswoman Sarah Huckabee Sanders said the poisoning did not come up in US President Donald Trump's phone call on Tuesday with Russian President Vladimir Putin.

Ms Huckabee Sanders said the call, in which Mr Trump congratulated Mr Putin on his re-election, was meant to discuss areas of cooperation between the two countries.

Where's the proof, Putin's spokesman asks

Photo: The British Consulate General building in St. Petersburg, Russia. (AP: Dmitri Lovetsky)

Mr Putin's spokesman, Dmitry Peskov, dismissed demands by Britain and its allies that Russia prove its innocence, saying that it was Britain which must provide proof.

Former Russian spy Sergei Skripal is the latest opponent of the Russian President to meet a mysterious demise.

"Let's stay sober-minded and first of all wait for proof from Britain" that Russia was to blame, he told reporters.

Mr Rink said Britain has refused to provide a sample of the agent it said was used in the poisoning because tests would reveal that it had not come from Russia.

He said each lab has its own chemical "signature," allowing experts to trace its origin.

"It would be immediately clear that it wasn't 'cooked' in Russia," he said.

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The Russian Foreign Ministry accused Britain and other EU member states of developing similar nerve agents, and said the British Government was stirring up "media hysteria" around the case to distract attention from troubles in negotiating the country's exit from the EU.

"Instead of cooperating with us … they are simply slinging mud at us," Deputy Foreign Minister Sergei Ryabkov said.

"A great country that has fallen so low only causes pity."

Ministry spokesman Maria Zakharova also emphasised that Britain had failed to provide any proof of Russian involvement, saying it had limited itself to "boorish accusations".

She said the Russian Foreign Ministry was summoning ambassadors on Wednesday to present Moscow's arguments and raise questions about the case.

AP

Topics: security-intelligence, defence-and-national-security, unrest-conflict-and-war, terrorism, russian-federation, england, united-kingdom

First posted March 21, 2018 11:41:07