Passengers are facing disruption at Heathrow Airport because of a problem with the runway lights.
An airport spokesperson said a technical issue was being investigated and disruption was anticipated throughout the morning.
British Airways said in a statement that it was diverting flights due to arrive at Heathrow shortly and delaying flights which had not yet departed.
Passengers are being advised to check their flight status with their airline.
People on planes due to fly into Heathrow have been reporting that they haven't been allowed to take off.
Heathrow operates at nearly 100% capacity, which means there is no space in the schedule to accommodate a number of delayed flights and so some services may need to be cancelled.
This will be agreed with air traffic control and the airlines operating at Heathrow, who will then pass this information onto passengers.