Stansted Airport's terminal building was evacuated after smoke from an air conditioning duct triggered an alarm.
Smoke was seen coming from an air conditioning duct
A spokesman at the Essex airport said passengers were asked to leave the building as a precaution but were allowed back after about an hour.
He said firefighters carried out an investigation but found nothing.
"A fire alarm was activated so we decided to evacuate the terminal but aircraft continued to land," the spokesman added.
Operations at the airport have now returned to normal.