A third runway could mean an extra 200,000 flights a year over London.
Air traffic control officials have denied suggestions Heathrow would have to close if a new airport was built in the Thames Estuary.
London Mayor Boris Johnson has championed a new airport as an alternative to a third Heathrow runway.
NATS chiefs, who control UK commercial airspace, told MPs traffic would be hit at existing London airports.
But it would not kill off Heathrow - and expanding that airport was still the best answer to increasing demand.
A feasibility study into a new airport to the south-east of London, an idea first proposed nearly a decade ago, is due to be published later this month.
A cross-party committee of MPs, chaired by Labour MP Nick Raynsford, the former minister for London, has also been set up to examine the plan, although the government has described it as "irresponsible and disastrous" and the Conservative leadership have yet to give it their backing.
Appearing before the Transport Select Committee, officials from NATS - which provides traffic control services at 15 UK airports - questioned the impact of a new airport on current flight patterns.
Operations director Ian Hall said NATS would not be able to do any research on the likely impact until firm proposals were put forward but said he believed expanding Heathrow was preferable.
In the event of an entirely new airport being built, he said, neither Heathrow, Stansted or London City would be able to operate at full capacity and the UK would not get the "full benefits" of capacity available otherwise.
"What we are trying to do is to illustrate that by adding an additional piece of infrastructure into that very complex mix would have a direct impact on overall capacity [in London].
Conservative MP David Wilshire said NATS had previously suggested that a Thames Estuary airport would result in the closure of Heathrow, Stansted and London City airports.
The firm was now giving a "slightly less firm explanation", Mr Wilshire said, prompting suggestions from Committee chairman, Labour MP Louise Ellmann, that it had modified its views because of the controversy at Heathrow.
Mr Wilshire pressed Mr Hall on whether the new airport would result in the "curtailment or complete closure" of other London airports "or something in-between".
Mr Hall denied that NATS had shifted its stance, saying it had been "consistent" in its approach on the issue of new airport capacity and had not been subject to any political pressure.
"I think we have also been consistent in our view that it is much better to expand existing infrastructure than it would be to create new infrastructure."
He added: "I remember saying that if you put an airport here and an airport here and the departures from that airport are flying straight towards the flights arriving from the other airport that something would have to give."
A coalition of local residents, councils and environmental groups have said they will take legal action to try and block a third runway at Heathrow.
Ministers gave the go-ahead for the new runway last month with conditions on the number of flights and type of aircraft that are able to use it.
The London Mayor has said the pollution from a vastly expanded Heathrow would damage the living standards of millions of residents of the capital and alternative ideas must be looked at.