The survey of 100 departures from Heathrow on Wednesday showed that not one of the flights left on time and 14% were delayed by more than an hour.
Live flight information boards on the website of Heathrow operator BAA, showed both long-haul and local flights were delayed by up to two-and-a-half hours.
A spokeswoman said there was a variety of reasons for the delays. “On this particular day, air traffic control chose to slow down aircraft around Heathrow because of high winds.
“However, it is very difficult to pinpoint the causes of delays as there are hundreds of reasons, from a passenger arriving late to the terminal to overcrowding at the airport.”
British Airways had the most delayed flights, with four out of 10 late flights being operated by the airline.
The most severe delay was a British Airways flight to Newcastle, which took off two hours and 38 minutes late – a delay that was longer than the flight time. A flight to Delhi left two hours and 23 minutes behind schedule.