Reference: Regulation (EU) No 965/2012 on Air Operations, associated Decision AMC/GM to Part CAT: AMC1 CAT.IDE.A.205
In the first instance, passengers wishing to use their car seat as a child restraint device (CRD) on board an aircraft should contact the airline or travel agency through which the ticket has been booked, in order to obtain information on which CRDs are acceptable and also to ensure that the car seat can be installed properly on the seat of the particular aircraft with which they will fly.
Since 2008, EU law determines that children under 2 years of age must be secured by an approved child restraint device when flying with EU airlines. This can either be a child seat or a 'loop belt', the latter being attached to the seat belt of the adult who is holding the child on his/her lap.
Regarding the use of a car seat as an acceptable child restraint device, this is regulated under the Regulation on Air Operations, through Annex IV (Part CAT), specifically CAT.IDE.A.205. The basis for the provision on child restraint devices is JAR-OPS 1 guidance material.
To conclude, the proposed examples for acceptable child restraint devices include car seats, and item 2 of AMC1 CAT.IDE.A.205 lists the standards with which the CRD should comply. This includes the UN standard ECE R 44, -03 or later series - this is commonly indicated on the car seat's label.