More than one in five British Airway flights are being switched between terminals at Heathrow Airport from Sunday.
Passengers are urged to make sure they go to the right terminal
The flight switch programme is aimed at making it easier for passengers to make transfers without changing terminals.
In the move, five destinations will now fly from Terminal 1 instead of Terminal 4 and seven destinations will move the other way.
BA has said passengers should make sure they know which terminal to go to.
In the past, long-haul passengers could have assumed they were flying from Terminal 4 and those travelling to the continent knew they could go straight to Terminal 1.
NEW TO TERMINAL 1
Now passengers will need to check their tickets to make sure they are going to the correct terminal.
BA said the switch would make it easier for many of the airline's transfer passengers to connect between long-haul and short-haul flights without changing terminals at the airport.
Martin George, director of marketing for BA, said people should call the airline or visit its website if they were unsure of where to go.
NEW TO TERMINAL 4
"It is important that passengers check which terminal they are flying from before they set off.
"We're doing everything we can to make information as widely available as possible for our customers."
It is the second and final phase of the airline's Heathrow flight switch programme.
The first phase the saw BA's two daily Tokyo Narita and two daily Johannesburg services transfer from Terminal 4 to Terminal 1 at the end of October 2003.