Heathrow Airport's Terminal 3 was evacuated following reports of a fire alarm on Tuesday.
Passengers were told to leave the building shortly before 1pm after alarms went off throughout the terminal.
Bystanders reported hearing sirens and seeing police cars arriving at the scene.
An airport spokesman confirmed the alert was a fire alarm and said it was "investigating this as a matter of urgency”.
One traveller reported that, during the alert, only security personnel were being allowed to enter the building and said that armed police were in the area.
The alarms stopped just after 1pm but passengers were still being asked to wait outside.
Heathrow tweeted: "Our teams have investigated and there is no fire in T3 despite the alarm. Passengers will be directed back into the terminal shortly."
Passengers reported being stuck on the runway following the alert and others were stopped from boarding planes as the incident was investigated.
One person reported scenes of “chaos” and said it showed the airport could not be evacuated safely in the event of an emergency.
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