Iberia and British Airways on Tuesday formally agreed details of a new corporate holding structure.

Iberia and British Airways which began talks almost two years ago, signed off on a 91-page merger plan document before filing it with Spain’s stock market regulator.

The latest step in the drawn-out and often fraught process follows the signing of a definitive merger agreement in April this year. The airlines say the planned deal will produce an airline with a 408-strong fleet which will carry more than 58m passengers a year to 200 destinations.

The legal structure of the combined entity is complex, mainly to allow both carriers to preserve their respective brands and international flying rights.

Iberia said on Tuesday its existing assets and liabilities would be pushed into an operating subsidiary called Iberia Operadora. The original entity, with the share capital, would then be merged with British Airlines Holdco SA, British Airline’s new Spanish vehicle, and with International Consolidated Airlines Group (AIG), which will be the holding group for the merger.

Shares in AIG would be exchanged for existing stock in BA at a ratio of one for every one held and in Iberia at one for every existing 1.0205.