The number of premium passengers flying with British Airways and Iberia dropped by 2% last month.
The early timing of Easter and strikes at the Spanish carrier were blamed for the decline by parent company International Airlines Group.
Non-premium traffic increased by a marginal 0.5% in the month.
Iberia cabin crew and ground staff unions went on strike for five consecutive days in March resulting in the airline only operating 64% of its flights.
Underlying market conditions remain unchanged from those described at the publication of fourth quarter results in February, IAG said.