Amber Rudd has said more is known about the substance that put caused former double agent Sergei Skripal and his daughter Yulia into intensive care.
She told Sky: "We do know more about the substance and the police will be making a further statement this afternoon to share some of that."
The home secretary was speaking following an emergency Cobra meeting this morning, in which senior ministers were updated about the "unusual" circumstances.
Skripal and his daughter Yulia are still fighting for their lives after being found unconscious on a bench in Salisbury on Sunday.
Two police officers were treated in hospital for minor symptoms, believed to include itchy eyes and wheezing, before they were given the all clear.
A third member of the emergency services remains in hospital.
This morning's Cobra summit came less than 24 hours after Theresa May convened a meeting of her National Security Council, which was attended by several senior ministers and is thought to have included heads of the UK's intelligence services.
The Prime Minister this afternoon told MPs "further such meetings" would take place this afternoon.