The fire alarm on the airfield meant several flights were unable to land or take-off from the airport on Wednesday.
Flights were forced to divert to Gatwick and Stansted airports, while some had to return to their original destinations after take-off.
Heathrow Airport said the alarm had been investigated, and “operations” had resumed.
A spokeswoman for the National Air Traffic Services said “the control tower had been evacuated” after an alarm was triggered.
Passengers tweeted about being stuck on planes because of the delays.
Source: BBC News