Flights from Luton Airport were delayed for longer than any other UK airport last year, analysis of Civil Aviation Authority data suggests.
Planes left 19.7 minutes late on average, with Gatwick, Jersey and Durham Tees Valley next worst.
The top performers were Heathrow - flights were 11 minutes late - Leeds Bradford, Belfast City and London City.
Scheduled and charter flights, but not cancelled services, from the 25 busiest airports were examined in the study.
The Press Association, which compiled the departure punctuality ranking, said flights across all airports left an average of 15 minutes late.
'Outdated' airspace infrastructure
The CAA said the data allows passengers to "make informed choices about which airports they fly from".
But a spokesman for the Airport Operators Association said "outdated" airspace infrastructure limits the efficiency of flights.
He said: "Airports are working with air traffic service providers and the government to plan and deliver the necessary changes so everyone can continue to fly with a minimum of delays."
A spokeswoman for London Luton said factors outside its control had an influence on punctuality including air traffic control strikes, late arriving aircraft, bad weather and congested airspace.
Gatwick said it was doing "everything within its power" to improve the proportion of its flights that depart on time, including using new technology to predict and recover from late running flights.
Worst airports for punctuality
Luton - 19.7 minutes
Gatwick - 18.9 minutes
Jersey - 18.7 minutes
Durham Tees Valley - 18.6 minutes
Birmingham - 18.2 minutes
Bristol - 17.6 minutes
Belfast International - 16.7 minutes
Manchester - 16.7 minutes
Stansted - 16.7 minutes
Aberdeen - 16.1 minutes
Edinburgh - 15.8 minutes
Cardiff - 15.7 minutes
Glasgow - 15.0 minutes
Doncaster Sheffield - 14.3 minutes
Liverpool - 14.0 minutes
London Southend - 13.5 minutes
Southampton - 13.5 minutes
East Midlands International - 13.3 minutes
Newcastle - 12.8 minutes
Bournemouth - 12.3 minutes
Exeter - 11.9 minutes
London City - 11.6 minutes
Belfast City - 11.3 minutes
Leeds Bradford - 11.3 minutes
Heathrow - 11.0 minutes
Average delay times in 2017 based on Civil Aviation Authority data