British Airways is advising all passengers who have a flight booked for today (Tuesday, July 22, 2003) to check-in as normal.
The airline expects to operate a full schedule today.
More than 520 flights in and out of Heathrow are scheduled to operate although there may be some delays on longhaul services from Terminal 4.
To minimise congestion at Terminal 4, flights are not opening for check-in more than three hours before departure and passengers are only being allowed into the terminal on a flight by flight basis.
Passengers from cancelled services who do not hold a reservation for a flight today are urged to call the special helpline 0800 727 800 or 0870 850 9850 to re-book their ticket.
Teams of volunteers are working around the clock to get services back to normal.
Yesterday, British Airways operated almost a full schedule out of Heathrow.