BAA said several airlines had been affected Thunderstorms and high winds have led to the cancellation of 70 flights in and out of Heathrow.
Airport operator BAA said 50 departures and 20 arrivals were cancelled on Thursday due to bad weather.
The flights were mainly scheduled between 1600 and 1900 BST on Thursday.
A BAA spokesman said delays are continuing, mainly to departures, due to the high winds and a backlog caused by the earlier disruption.
He said: “Due to localised thunderstorms and high winds, unfortunately we had to cancel some flights affecting a number of airlines.
“There are some continuing delays at the moment and we cannot give a timescale on when things will completely be back to normal.”
He advised passengers to continue to check for updates.
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