Heathrow will need to close if the government decides to expand Stansted or build a new Thames estuary airport, according to a transport think tank.
The Independent Transport Commission said a major capacity airport is needed to compete with European rivals.
It said Heathrow would have to close to give investors confidence that airlines would move their business.
The Flying into the Future report says: "In the event of a decision to develop a major hub airport at either locations [Stansted or the Thames Estuary], we do not see how the current Heathrow could continue to operate.
"The majority of our respondents share this view."
It discounted the idea of a dual airport dubbed 'Heathwick', developing competing hubs across the UK or keeping the status quo at Heathrow.
A number of bodies - including airport protest groups and airport authorities - contributed to the report which will be submitted to the government's Airports Commission, the BBC reported.
Closing Heathrow would have "major impacts on the 114,000 people directly and indirectly employed by the airport as well as their families and the communities in which they live", it said.
The report added that releasing some 1,200 hectares of land - the size of Kensington and Chelsea - could offer "unparalleled opportunity for redevelopment for housing and other uses in a prime west London location".
But it does not believe enough consideration has been given to the possible loss of jobs or the potential cost of compensating airlines which have invested in Heathrow.
Spokesman Stephen Hickey said: "Heathrow has now been overtaken by many of its competitors on mainland Europe and that will be a loss to the UK and London.
"At the moment we have the benefit of one of Europe's top hub airports.
"The risk is we are losing that capacity to Charles de Gaulle, Frankfurt [and] Schiphol and the airlines will want to use those airports."
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