It follows after 30 people were treated for chemical inhalation during the incident, with 11 people taken to hospital yesterday (December 16) at 9.30am.
According to the London Fire Brigade, the chemical in question is suspected to be carbon monoxide.
Around 35 people were evacuated from the store during the incident and while an investigation takes place, the M&S store remains temporarily closed today (December 17).
The remaining 19 patients who were treated were discharged at the scene.
According to Greenwich councillor David Gardner, no customers are believed to have been affected from the leak.
It is claimed that six people now remain in hospital and are being treated, all of whom are staff members.
None are believed to be in a life-changing position, the councillor continued.
Fire crews carried out further sweeps to check for no elevated readings. They have also completed a ventilation of the building.
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 were called at 0835 today (16 December) to a suspected carbon monoxide leak at a shop on Gallions Road in Charlton.
“Firefighters wearing breathing apparatus have used specialist equipment to carry out a systematic sweep of the building and found elevated readings of carbon monoxide.
“Crews have carried out further sweeps to check for no elevated readings. They have also completed a ventilation of the building.
“Around 35 people left the building before firefighters arrived and crews have assisted colleagues from the London Ambulance Service to treat a number of patients at the scene.
“Two fire engines and two fire rescue units were sent to the scene.”
