The airlines group, which owns British Airways, Iberia and Ireland's Aer Lingus, launched Level on March 17, offering flights from Barcelona to the Americas in response to rising competition on transatlantic routes.

IAG's chief executive Willie Walsh said ticket sales in the first month had been "incredible" and the airline would be expanded over the next year.

"This is just the start. In summer 2018 we will have more aircraft and will operate more destinations from Barcelona," Walsh said in a statement. "We're also planning to expand Level operations to other European cities."

Level currently has just two Airbus A330s in its fleet, but IAG is looking to expand the service to fend off low-cost transatlantic offerings from the likes of Norwegian Air Shuttle (>> Norwegian Air Shuttle ASA) and Wow Air.

IAG's shares were up 3.5 percent, the second biggest rise on the FTSE 100 index, with other airlines across Europe lifted after Lufthansa (>> Deutsche Lufthansa AG) said pricing had improved and the German airline's stock was upgraded by Exane.

(Reporting by Alistair Smout; Editing by Mark Potter)