EasyJet is planning to stop passengers boarding flights if they arrive at the airport with less than 30 minutes to go to take-off.
The budget airline is introducing the change, telling customers with a warning on their boarding passes.
Anyone who won't get through security before the deadline will be turned away and miss their flight, or have to go back to an EasyJet desk to rearrange it.
The airline charges an £80 so-called "rescue fee" to make a change if you don't have insurance to cover a missed flight.
The new rules are being brought in at London's Gatwick Airport where automatic security gates have been changed to stop travellers going through with less than half an hour to go.
A spokesman for the company has said it's to improve punctuality for passengers.
The idea is being brought in after a similar scheme run by British Airways at Heathrow.