Nearly a third of passengers flying to or from London Airports were delayed last summer - with some 82,250 flights affected.
Up to 13 million people were hit by late flights airports in the capital alone, while nearly 19 million passengers could have been affected at airports across the country.
Of the London airports, Gatwick posted the worst figures - with 43 per cent of all flights arriving or leaving more than 15 minutes late and an average delay of 17 minutes, according to Civil Aviation Authority (CAA) figures.
The airport, which handled 70,900 passenger flights between July and September 2015, performed considerably worse than Heathrow, which managed to keep 71 per cent of its 122,900 scheduled flights on time.
The number of passengers at London airports in the third quarter of last year was higher than ever, but all except Stansted recorded a year-on-year decrease in on-time flights, suggesting they may have been struggling with the extra demand.
Tim Johnson, CAA Policy Director, said: "The first three quarters in 2015 have been the strongest for passenger numbers since records began.
“Airlines are accommodating the continuing strong passenger demand by carrying the extra passengers on larger aircraft, rather than increasing the number of flights significantly.
“However, punctuality was the worst of any summer period since 2010 - something we know many passengers will have found frustrating and an issue airlines, airports and air traffic control services should work to address.
“The strong passenger demand and a drop in punctuality also underlines the importance of addressing the on-going pressures on runway capacity in the South East and the need to modernise the UK's airspace.”
The five worst airports in the world
The five worst airports in the world
1/5 5: Jomo Kenyatta, Nairobi
Africa has some excellent hub airports, including Addis Ababa and Casablanca, but Kenya’s main gateway is not up to the job.
2/5 4: Geneva
Forget the stereotype of Swiss efficiency: the rapid growth of passenger numbers at the British traveller’s key Alpine airport has not been matched by much-needed investment.
3/5 3: Sheremetyevo, Moscow
Built for the 1980 Olympic Games, and heading downhill ever since.
4/5 2: Fiumicino, Rome
Even before the fire in May that cut capacity, the Italian capital’s main airport was a confusing shambles.
5/5 1: Charles de Gaulle, Paris
The layout of Continental Europe’s leading hub is bizarre, and even same-terminal transfers are awful at Air France’s base.
A Gatwick Airport spokesperson said: “Gatwick operates the world’s busiest single runway airport and strives to get all flights away on time but incidents beyond Gatwick’s control had a big influence on the airport’s overall performance and punctuality in Q3 2015.
“The on-time performance of airlines operating at Gatwick was affected by a large increase in congestion over European airspace affecting 30 per cent of flights during the summer period, as well as air traffic control strikes in Europe and periods of adverse weather across the continent.
“Gatwick has more flights to Europe than any UK airport and can therefore be impacted disproportionately by events affecting European airspace.
“Improving our performance in this area is a priority and we continue to work very closely with all airlines and other partners to ensure passengers are provided with the best experience that can be delivered.”
Outside of London, Leeds Bradford was the most punctual airport of last Summer, with 84 per cent of its 9,500 flights leaving or arriving on time, while Jersey was the worst with nearly third of all flights late.