A total of 30 people were treated at Gallions Road for chemical inhalation today (December 16) at around 9.30am.
According to the London Fire Brigade, the chemical in question is suspected to be carbon monoxide.
Hours later, 19 of the patients treated have been discharged at the scene, while 11 of them were taken to hospital.
Their condition is unknown at this time.
While an investigation takes place, the M&S store is temporarily closed.
James Johnson, Strategic Commander for the London Ambulance Service, said: “We were called at 9.32am today (16 December) to reports of a suspected carbon monoxide incident on Gallions Road, Charlton.
“We sent a number of resources to the scene including ambulance crews, incident response officers, paramedics in fast response cars, advanced paramedics and paramedics from our hazardous area response team (HART).
“We treated 30 people for chemical inhalation. We took 11 patients to hospital and discharged 19 patients at the scene.”
A London Fire Brigade spokesperson said: “Firefighters are currently at the scene of a suspected carbon monoxide incident at a shop on Gallions Road in Charlton.
“Crews are working alongside multi-agency partners to resolve the incident safely.
“The Brigade was called at 0835 and two fire engines and two fire rescue units are at the scene.”
A Marks & Spencer spokesperson said: “Our Charlton store is temporarily closed as we investigate an incident that happened earlier this morning, we hope to reopen as soon as possible.”
