The Londons Fire Brigade was alerted to reports of a suspected carbon monoxide leak on Gallions Road at around 8.30am (December 16).
Around an hour later, the London Ambulance Service was dispatched.
A hazardous area response team was among paramedic units in attendance at the scene, where 30 people were found in need of medical attention.
The M&S store on Gallions Road was taped off by the fire brigade whilst the scene was dealt with.
Of the 30 people treated at the scene, eleven patients were later taken to hospital for further treatment for suspected carbon monoxide poisoning.
Their conditions have not been confirmed at this stage.
The M&S store has been closed temporarily to allow for an investigation to carried out into the incident.
Firefighters wearing breathing apparatus entered the retail space, using specialist equipment to carry out a sweep of the branch.
According to LFB, crews found “elevated readings” of carbon monoxide and have since vented the building.
Crews were stood down by 1.50pm.
A spokesperson for LAS said: “We were called at 9.32am today (December 16) 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 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.”
