Major European airlines will "cancel" the Open Skies agreement if they do not get "more grip" on US carriers.
The warning came from Dr Pablo Mendes de Leon, professor in air and space law at Leiden University, The Netherlands.
He said unless stage two of the deal between the EU and the US succeeded in giving European carriers ownership rights in America, major European carriers would walk away from the agreement.
"There is a lot of pressure from European carriers. They will not allow the second stage unless it gives them more grip on US carriers.
"They are saying 'We want to establish our services there and if not, we are going to leave it (the deal) at stage one. We will cancel it.'
"This is a threat as well as a negotiating tool. You will have BA and Lufthansa saying 'if stage two is not going to happen, we will cancel it.'
"We would have to fall back on the old Bermuda agreement which will mean that Heathrow would again be closed," he said.
Prof Mendes was speaking to 50 delegates at an ACTE executive forum in Amsterdam this week.
He said that while stage one of the Open Skies deal comes into force on April 1, there were still major disagreements between the EU and the US.
Talks on stage two are due to begin in June and be successfully concluded by the end of 2009.
But besides ownership - US rules currently prevent foreign control of US-based carriers nor under the deal can foreign airlines operate domestic services in the US - there are also differences on security, air traffic management, aviation regulations and environmental policies.
A second aviation expert, Jaap de Wit of Amsterdam Aviation Economics told the meeting: "I would not be excited about it so far. It is not a big shift."
* The EC and the US Department of Transportation have launched a joint research programme on the "robust growth of alliances, the effect of alliances on airline competition and possible changes in the role of alliances" after the Open Skies agreement. The main findings will be published in mid-2009.
* see BTE Analysis