Is there a legislation that food additives have to be labelled with the E-number instead of the name only?

Austria

The ingredient list shall provide all relevant information to the consumer. With regard to food additives, community legislation on the labelling of foodstuffs says that the use of food additives must always be labelled on the packaging in the list of ingredients by their category (anti-oxidant, preservative, colour, etc) with either their name or E-number. Both, the name and the E-number have been set by law. Therefore both are unique for a certain additive and thus allow clear identification of the substance used. Consequently, the consumer can make its informed choice. Whether in practical terms, the name or the E-number is more informative to the consumer depends. For this reason the manufacturer is free to take its choice.

Detailed rules on labelling of additives in foodstuffs, and on additives sold as such to food producers and consumers are laid down in Community legislation (Directive 2000/13/EC, Regulation 50/2000/EC and Directive 89/107/EEC).
http://europa.eu.int/comm/food/fs/sfp/flav_index_en.html

The E-number signifies approval of an additive by the EU. To obtain an E-number, the additive must have been fully evaluated for safety. The E-number system also serves as a simple and convenient way to label permitted additives across the range of languages in the European Union.