International chemical weapons experts will arrive in Britain today, to test the nerve agent used in the poisoning of the former Russian spy Sergei Skripal and his daughter in Salisbury.
A team of up to ten experts from the Organisation for the Prohibition of Chemical Weapons (OPCW), which polices the prohibition of chemical weapons, will visit the Ministry of Defence's scientific research facility at Porton Down.
They'll try to work out how samples of the nerve agent used in the attack can be transported abroad, so they can be tested in independent labs overseas.
It's thought the results could take at least two weeks.
In Salisbury, Sergei and Yulia Skripal remain critically ill in hospital.
Following his re-election as President of Russia, Vladimir Putin dismissed claims of any Russian involvement in the Salisbury spy poisoning as "nonsense".
The leader claimed that Sergei Skripal and his daughter would have died instantly if they had been attacked with a nerve-agent.
Boris Johnson will update European Union counterparts on developments in the attack during a meeting in Brussels on Monday before holding talks Nato secretary general Jens Stoltenberg.
The Foreign Secretary has accused Russia of stockpiling Novichok for a decade in breach of international rules.
President Putin, who secured a fourth term amid widespread claims of electoral fraud, said he learnt about the "tragedy" from the media.
"The first thing that comes to my mind is that should it really be a warfare agent, people would have died instantly," he said.
"It is an obvious fact.
"Russia does not possess such agents. We have destroyed all our chemical arsenals under control of international observers."
"We are ready for cooperation and said that immediately.
"We are ready to take part in all necessary probes but the will of the other side is needed for that. So far, we see none."
Moscow mocked the UK, claiming the government's response had fuelled an increase in support for President Putin in the election.
The Foreign Secretary said Russia has investigated ways of delivering nerve agents for assassination over the last 10 years in a breach of international rules.
Russia's ambassador to the EU, Vladimir Chizhov, suggested the Novichok may have come from the Porton Down laboratory, which is about eight miles from Salisbury.
Sweden and the Czech Republic denied Russian suggestions they may have been the source of the nerve agent.
The national security council will meet in the coming days to discuss Moscow's tit-for-tat response to the UK's expulsion of 23 Russian diplomats.
Mr Skripal, a former double agent, and his daughter may have been exposed to a deadly nerve agent through his car's ventilation system, US media has reported.
The pair are still fighting for their lives after being exposed to Novichok two weeks ago in the Wiltshire city.
Counter-terrorism police renewed their appeal for sightings of Mr Skripal's burgundy BMW 320D saloon car, registration HD09 WAO, in Salisbury on the morning of Sunday March 4.
Metropolitan Police Assistant Commissioner Neil Basu said:
"We are learning more about Sergei and Yulia's movements but we need to be clearer around their exact movements on the morning of the incident."