Air passengers were warned that the "technical issue" affecting the lighting at runways in Heathrow would affect their travels on Wednesday.
The UK's largest airport said it was trying to fix the problem, but recommended passengers to check if they had detected by plane whether their flights were involved.
A spokeswoman for Heathrow said: "Our engineering teams are investigating a technical issue with the lighting system for the airport runways.
"We have activated our emergency plans and both runways are currently open and operational, although we expect a few delays at the airport this morning.
"For the latest information, passengers must check their flight status with their airlines before traveling to the airport.
"We apologize for any influence that we may have on our passengers."