The owner of Heathrow Airport said today 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."
But Liberal Democrat transport spokesman Norman Baker said: "This is a worthless promise, as BAA has to stick to EU emissions limits anyway.
"It is clearly an attempt to get the new runway in place and then to come back demanding more flights at a later date.
"The last 15 years have been littered with promises of no more expansion, followed by demands for exactly that."
Wandsworth Council leader Edward Lister, speaking on behalf of the 2M Group of councils opposed to Heathrow expansion, said: "The history of Heathrow is littered with broken promises.
"Once extra capacity has been agreed, it is used to the full. No-one will believe claims by either BAA or the Government that flights will be cut in the future in the light of environmental concerns.
"The time for an independent review is now. Given criticisms by the Government's own environmental advisers of the case made by the Department for Transport (DfT) on noise, air quality and surface access, there should be an immediate halt to any expansion proposals while the science is thoroughly checked.
"EU air quality limits are already being exceeded at Heathrow. If BAA and the DfT seriously want to be trusted on the environment they can start by reducing flights today."