Heathrow airport is on lock as as passengers are evacuated from Terminal Four following a fire alarm.
Footage from the scene shows the mass confusion as passengers try to find out how to leave the terminal, but staff are unable to provide them with details.
People carrying hand luggage and suitcases can be seen milling through the building with no sense of urgency as they try to find out what is happening.
Harriet Constable, who is at the scene, tweeted images of huge crowds of people trying to flee the terminal but getting stuck at a queue at the front doors.
A spokesman for the airport says the incident is being looked into and they are aware of the evacuation, but could give no further details at this time.
The evacuation comes just a day after the airport, in west London saw its Terminal Three also cleared of passengers in a similar alert.