Police are investigating the suspected poisoning of a former Russian spy and his daughter.
Earlier, Home Secretary Amber Rudd said police had more information about the substance.
Speaking after chairing a COBRA meeting, she said: “This is likely to be a lengthy investigation.
“We need to make sure we respond to evidence, not rumour.
“We do know more about this substance. We must let the police carry on this work, and they will share more information this afternoon.
“There is a lot of information about him (Mr Skripal) but I am not going to comment further about that.”
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