Five out of six people would oppose building a new airport in the Thames Estuary if it meant closing Heathrow and other airports, a survey has found.
An estuary airport on the Isle of Grain and the closure of Heathrow has been proposed by London Mayor Boris Johnson.
The online survey of 2,000 adults from across the UK was commissioned by Medway Council and Kent County Council (KCC), which oppose the new airport.
Mr Johnson's office has not yet commented on the survey.
The mayor is due to submit final plans for the estuary airport to the Airports Commission on Friday.
The commission, which is considering how to expand air capacity in south-east England, has drawn up a shortlist which comprises building a third runway at Heathrow, lengthening an existing runway at Heathrow, and building a new runway at Gatwick.
It is also considering the £148bn Isle of Grain airport in north Kent.
Mr Johnson has said if the new airport is built, a city of 190,000 homes and thousands of jobs could be created on the Heathrow site.
Medway and KCC believe City and Southend could also close, but Mr Johnson has not said this would be the case.
The survey found 38% of those asked supported an estuary airport. But when they were told Heathrow, City and Southend airports could close as a result, the support dropped to 16%, or just over one in six.
"We strongly believe that plans to site a hub airport in the Thames Estuary are financially, geographically and environmentally wrong," said Rodney Chambers, leader of Medway Council.
"It will waste tens of billions of pounds of taxpayers' money for a project which is on the wrong side of London for the majority of passengers.
"This survey highlights the chronic lack of public support for the project.
"Most people will not back a scheme which closes other key airports including Heathrow and City and Southend Airports."
The Airports Commission will make its final recommendation on airport expansion in summer 2015.