Aer Lingus has rejected a takeover bid by British Airways parent International Airlines Group (IAG), the Irish flag carrier confirmed on Thursday. In a statement, Aer Lingus said it received a “preliminary, highly conditional and nonbinding approach from IAG” on December 14, and that it rejected the bid on December 16.

“The board has reviewed the proposal and believes it fundamentally undervalues Aer Lingus and its attractive prospects,” the statement read.

Aer Lingus said it issued the statement in reaction to “media speculation” and an confirmation by IAG on Thursday that it considered a possible offer for the Irish airline.

Shares in Aer Lingus rose by more than 20 percent on Thursday after the Financial Times first reported on the possible takeover effort.

 “International Consolidated Airlines Group notes the recent movement in the share price of Aer Lingus and confirms it submitted a proposal to make an offer for the company, which has been rejected by the board of Aer Lingus,” said IAG in its own statement.

Long considered a potential takeover target of IAG, Aer Lingus controls a substantial number of coveted slots at congested London Heathrow Airport. It also has seen its financial fortunes improve lately, to the point where last month it increased its profit forecast for the year amid a recovery in the Irish economy and increases in long-haul traffic. Last year, the European Commission obliged rival carrier Ryanair to reduce its stake in Aer Lingus from 29.8 percent to 5 percent, arguing that the holding was anti-competitive.