Belfast International Airport might claim for damages amounting to £40m, double the initial claim, against Irish carrier Aer Lingus after the latter moved to a rival airport.
The airport accused the carrier of a breach of contract over its move to Belfast George Best City Airport in 2012.
The Irish News had revealed in November that the airline was being sued for £20m in what was then believed to be the biggest claim for breach of contract in Irish aviation history. The two airports are slated to meet in court in Belfast in April.
Aer Lingus is currently being eyed by BA owner International Airlines Group (IAG) for acquisition.
Aer Lingus had signed a ten-year contract with Belfast International Airport in 2007 to fly from the airport. However, after five years, the previously state-owned carrier terminated its contract and moved to the George Best City Airport. The carrier runs daily flights from the airport to London Heathrow and Gatwick airports.
According to a report in the Guardian, air industry sources said that the airport's lawyers brought in independent forensic accountants to analyse the losses because of Aer Lingus' move.
The Guardian quoted Aer Lingus as saying: "Aer Lingus is not willing to comment on the detail of this case given that a hearing is pending; however, they are strenuously contesting the claims made by Belfast International Airport."
Aer Lingus' switch was followed by the closure of the BMIbaby service at George Best City airport to London, which would carry close to 400,000 passengers on the route annually.
The revelation has come in the same week as Aer Lingus appointed a new chief executive officer, Stephen Kavanagh.