Aer Lingus is believed to be planning to switch its operations from Belfast International Airport to Belfast City Airport.
It follows the City's loss of BMI Baby - which carried 400,000 passengers a year. The airport is expected to confirm details of new airlines soon.
Aer Lingus flies to Heathrow and seven European destinations, but could expand routes to regional UK airports from the City, filling some of the lost BMI Baby routes and competing with Flybe. Belfast International Airport made no comment.
A spokesman for George Best Belfast City Airport said: "Following the announcement of the departure of BMI Baby we stated that we were confident that we would attract new airlines. This position has not changed and we expect to make several announcements in the coming weeks. We are certainly not in a position to comment on speculation regarding specific airlines at this stage."
A formal announcement may come in days. An Aer Lingus spokeswoman said: "It is pure speculation and no such decision has been reached."
The move would make George Best Belfast City Airport Belfast's only Heathrow connection. With the takeover of BMI by British Airways there had been concerns over that route.
BA has confirmed its commitment to the route but there remains speculation that there may be long-term plans for both airlines to work together on the Belfast-Heathrow connection.
DUP MP William McCrea, who represents the South Antrim constituency where the International Airport is based, said the move would be a retrograde step.
"I cannot understand why Aer Lingus would find it acceptable to compete with British Airways for passengers from the City Airport leaving a vast geographical area of the province without service to Heathrow," he said.
"I accept the removal of BMI Baby was a real blow to the City Airport but seeking to rob airlines from the International Airport rather than attracting new airlines to the province does not seem sustainable."