BA's long-haul flights will move to Terminal 5 in June
People travelling from regional airports and transferring to long-haul flights could face disruptions because of the problems at Terminal 5.
British Airways has delayed moving its long-haul flights from Terminal 4 to Terminal 5 until June.
But the summer schedules were designed assuming that passengers would depart from the same terminal they arrived at.
BA said it would contact any passengers affected and offer them a refund or an opportunity to rebook.
A BA spokesperson said that only a minority of passengers would be affected.
If passengers have to move between terminals for a transfer then their minimum transfer time goes up from one to two hours.
"If you're flying Manchester to Delhi, if you'd got the 0745 flight out you could have expected beforehand to be in Delhi at 11 o'clock at night," said Simon Calder, travel editor of the Independent.
"Now you're not going to get there until six the following morning because you've got to get across from Terminal 5 to Terminal 4."
The disruption could affect people travelling between 30 April, when the long-haul flights were supposed to be moving to Terminal 5, and whatever date in June the switch actually takes place.