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Police were not told a firework had been let off at a service station prior to its evacuation.
Kent Fire and Rescue Service say two women were treated for smoke inhalation after a "small indoor pyrotechnic firework" was set off at the M20 service station at Junction 8, near Hollingbourne.
Firefighters were called at 8.21pm and searched the building using breathing apparatus.
Police say they were called to organise the closure of the service station following reports of toxic fumes coming from the service's arcade area.
However, they had no knowledge of a firework being set off.