John Stewart, chairman of the anti-Heathrow expansion group HACAN, said David Cameron 's resignation and widespread predictions that Boris Johnson will be the next prime minister have seriously dented the airport's hopes of a third runway.
"It's now much less likely there will be a third runway than it would have been had we voted to remain in the EU," said Mr Stewart.
"People like Boris Johnson are the winners of this referendum. If he becomes prime minister or has a big post in a new government it's much less likely there will be a third runway.
"It's also very unlikely there will be a July decision. I now expect an announcement this autumn."
Mr Stewart added that the referendum result also meant the likely resignation of Chancellor George Osborne, viewed by many pundits as being pro-Heathrow expansion, from the Government.
'Case for expansion now stronger than ever'
Heathrow said the outcome of the referendum made the case for a third runway "stronger than ever".
A spokesperson for the airport said: "With today’s result, the case for expansion at Heathrow is stronger than ever before.
Only Heathrow can help Britain be the great trading nation connecting all regions of the UK to the world. It is the keystone that connects businesses of every size to markets across the world as the UK's only global hub airport.
"Global connections are critical for a new outward-looking UK to help our businesses and economy to thrive - and with expansion we can deliver up to 40 new destinations on top of the 83 we serve now.
"We are confident that the Government will make the right choice for the future of the UK, putting the interests of the country first."
"We look forward to working with the Government and its agencies on next steps."
Heathrow had been clear favourite to get a new runway
Heathrow had been a clear favourite to get a new runway, ahead of Gatwick, since the Airports Commission last summer recommended a third runway should be built there.
Mr Johnson has campaigned vociferously against a third runway , along with fellow Leave campaigner Zac Goldsmith.
The Uxbridge and South Ruislip MP has instead voiced his support for a new airport, dubbed Boris Island, which would be built in the Thames Estuary.
We will be getting more reaction throughout the day to the referendum result and what it means for Heathrow and the country as a whole.
You can follow all the action on our live blog .