The owner of Heathrow Airport said it would ask the Government to appoint an independent assessor to check noise and air quality limits if it received the go-ahead for a third runway at the site.
A spokesman for BAA said it would only increase the number of flights at the airport if it can operate within these limits.
BAA chief executive Colin Matthews said: "We have listened to the many arguments around expansion at Heathrow.
"Although the economic case remains compelling, we understand that we can only increase the number of flights if we can safeguard levels of noise and air quality.
"By calling on an independent assessor to scrutinise the airport's performance against these limits, we are providing an uncompromising assurance that we will operate Heathrow Airport within the limits laid down by government.
"If we don't, the number of flights in and out of Heathrow could be capped."
