National identity cards in the European Economic Area

National identity cards are issued to their citizens by the governments of all European Union member states except Denmark, Ireland and the United Kingdom, and also by Liechtenstein and Switzerland (the latter not formally part of the EEA). Citizens holding a national identity card, which states EEA or Swiss citizenship, can not only use it as an identity document within their home country, but also as a travel document to exercise the right of free movement in the EEA and Switzerland. Identity cards that do not state EEA or Swiss citizenship, including national identity cards issued to residents who are not citizens, are not valid as a travel document within the EEA and Switzerland.

National identity cards are often accepted in other parts of the world for unofficial identification purposes (such as age verification in commercial establishments that serve or sell alcohol, or checking in at hotels) and sometimes for official purposes such as proof of identity/nationality to authorities.

Four EEA member states do not issue cards defined by EU as national identity cards to their citizens: Denmark, Iceland, Norway and the United Kingdom (except to residents of Gibraltar); although Norway is expected to start issuing such cards from 1 April 2018. At present, citizens from these four countries can only use a passport as a travel document when travelling between EEA member states, and Switzerland. However, when travelling within the Schengen Area or Common Travel Area, other valid identity documentation (such as a driving licence or EHIC card) is often sufficient.Ireland issues a passport card which is valid as national identity card in other EU countries.

...
Wikipedia