The world’s chemical weapons watchdog is to meet in the Hague and discuss the poisoning of a former Russian spy and his daughter in the UK.
The emergency session was called by Russia, who denies being behind the attack and wants the UK to share evidence.
But the UK government says the only “plausible explanation” is that Russia is to blame.
The incident has sparked a deep diplomatic rift.
On Tuesday the UK’s Porton Down laboratory said it could not verify the precise source of the nerve agent used against Sergei Skripal and his daughter Yulia.
The laboratory, which has previously identified the substance as a military-grade Novichok nerve agent, said it was likely to have been deployed by a “state actor” but said it was not their job to say where it was manufactured.
Porton Down’s chief executive Gary Aitkenhead dismissed Russian claims it might have come from the UK military laboratory.
The UK says further intelligence led to its belief that Russia was responsible.
What is the meeting about?
As a member of the Organisation for the Prevention of Chemical Weapons (OPCW), Russia has the right to request an emergency meeting of the body.
Among other things, it wants to know what kind of evidence the UK has provided to the OPCW, which inspectors visited the site of the attack in Salisbury, who they met and where the samples are being analysed.
Speaking in Turkey, Russian President Vladimir Putin said that while Moscow hoped to draw a line under the affair, it would still play a role in the inquiry into what happened.
However, the UK Foreign Office has called the meeting a “diversionary tactic, intended to undermine the work of the OPCW in reaching a conclusion”.
“There is no requirement in the chemical weapons convention for the victim of a chemical weapons attack to engage in a joint investigation with the likely perpetrator,” a spokesman said.
The OPCW expects to receive the results of its own independent laboratory tests within a week.
It does not have the power to attribute blame, but it could ask the Kremlin to grant their inspectors access to former Soviet Union production facilities to check all of their chemical weapons stockpiles have been destroyed.
What happened in Salisbury?
Sergei Skripal and his daughter Yulia were attacked with the nerve agent on 4 March.
The BBC understands Miss Skripal, 33, is now conscious and talking. Salisbury District Hospital has said her father, 66, remained critically ill but stable.
Mr Aitkenhead said he had been advising those treating the Skripals.
“Unfortunately this is an extremely toxic substance. There is not, as far as we know, any antidote that you can use to negate the effects of it,” he added.
What has been the reaction so far?
So far 29 nations have expelled diplomats over the poisoning.
Russia has now told the UK that more than 50 of its diplomats have to leave the country.
In a news conference on Monday, Russian Foreign Minister Sergei Lavrov suggested the poisoning could be “in the interests of the British government” because of the “uncomfortable situation” they had found themselves in with Brexit.
“There could be a whole number of reasons and none of them can be ruled out,” Mr Lavrov said.