"It is crucial that our plans maximise the benefits of expansion across the country, including for the communities closest to us", said Emma Gilthorpe, Heathrow's Executive Director for Expansion. "We apologize to passengers for any inconvenience this may cause".
Police Commander Stuart Cundy said police officers were among those who had seen the drone.
The drone was spotted over Heathrow this evening and sparked chaos as dozens of flights were suspended.
The incident at Gatwick happened a few days before Christmas and marked the first time such a thing had happened anywhere in the world.
Arriving planes are continuing to land at Heathrow.
Heathrow and Gatwick reportedly spent millions of pounds on "military-grade anti-drone equipment" after the previous drone sighting scandal, which is still being investigated.
The military is helping police after sightings of a drone temporarily halted flights at Heathrow airport on Tuesday.
While that would boost punctuality, "flight paths could overfly areas that are not affected by Heathrow arrivals today", the airport said in the study on proposed airspace changes.
Flight tracking website Flightradar24.com showed many aircraft circling around the west London airport.
Coincidentally, this comes on the same day the United Kingdom government's Department for Transport announced a series of new drone-related powers for police, including new airport exclusion zones.
The Home Office has said it will begin testing new counter-drone technology to allow airports to detect them and bring them down if they pose a threat.
Ministers also announced that from November 30, operators of drones weighing between 250g and 20kg will be required to register and take an online drone pilot competency test.