Two officers were also exposed to the substance and remain at Croydon University Hospital, formerly Mayday Hospital.

Officers were alerted at 3.46am by London Ambulance Service to reports of a woman having ingested poison at an address in Thornton Heath, the Metropolitan Police said in a statement.

"The woman was taken to a south London hospital by LAS where she was found to be in possession of a possible hazardous substance," the force said.

"Officers and London Fire Brigade attended Mayday Hospital and safety cordons and closures were established while specialist officers examined the item and safety procedures were enacted."

"Two officers were exposed to the substance. They remain at the hospital under observation," it added.

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Ambulances and emergency vehicles were given access to the site while emergency services responded to the incident.

Road closures have since been lifted and the incident stood down, police said, adding there were no arrests.

A spokesperson for Croydon Health Services NHS Trust said: "Following an incident in Croydon this morning we have treated a small number of patients, including two police officers, for chemical exposure.

"This was an isolated incident and Croydon University Hospital has remained open for patients throughout."