The new dimensions are larger than the recently agreed European minimum bag size of 40 x 30 x 15cm.
This adjustment ensures that existing Ryanair-compatible cabin bags will remain usable for passengers.
Rival airlines, including easyJet and Wizz Air, will maintain their current cabin baggage dimensions.
Airlines for Europe is working with the EU transport commissioner to simplify baggage rules, while major low-cost carriers oppose a recent MEP vote for a mandatory two-bag limit.