No guarantees on nerve agent culprits or finding more nerve agent Novichok: UK police

Those behind nerve agent poisonings in England have not been identified and police cannot guarantee there are no more traces of Novichok still in England, Britain's top counter-terrorism policeman said on Wednesday.

Former Russian spy Sergei Skripal and his daughter were poisoned with the Novichok nerve agent in Salisbury in March, and two Britons fell ill this month after exposure to the same nerve agent. One those, Dawn Sturgess, has died, while the other is critical but conscious.

"I would love to be able to stand here and say how we have identified and caught those responsible and how we are absolutely certain there are no traces of nerve agent left anywhere in the county," Assistant Commissioner Neil Basu said in a statement.

"The brutal reality, however, is that I cannot offer you any such assurances or guarantees at this time."

He added that it may never be possible to definitively establish a link between the death of Sturgess and the poisoning of the Skripals.

Prime Minister Theresa May said she was "appalled and shocked" by the death of Sturgess. "Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder," she said.

She offered her condolences to the relatives and loved ones of Sturgess, 44, who had three children. She became ill alongside a man named locally as Charlie Rowley, 45.

They were believed to have become exposed to Novichok by handling a container, and a link to the Salisbury attack in March is a main line of investigation.

That incident left former Russian spy Sergei Skripal and his daughter Yulia, who was visiting from Russia, in a critical condition although they later recovered.