Passports of the European Economic Area

Passports issued by European Economic Area member states (the European Union, plus Iceland, Liechtenstein, and Norway) or Switzerland can be used by citizens to exercise the right of free movement within the European Economic Area and Switzerland.

When going through border controls to enter an EEA country, EEA and Swiss citizens possessing valid biometric passports are sometimes able to use automated gates instead of immigration counters. For example, when entering the United Kingdom, at major airports, adult holders of EEA or Swiss biometric passports can use ePassport gates, whilst all other EEA citizens (such as those using a national identity card or a non-biometric passport) and non-EEA citizens must use an immigration counter. Anyone travelling with children must also use an immigration counter.

As an alternative to holding a passport, EEA or Swiss citizens can also use a valid national identity card to exercise their right of free movement within the EEA and Switzerland. Strictly speaking, it is not necessary for an EEA or Swiss citizen to possess a valid passport or national identity card to enter the EEA or Switzerland. In theory, if an EEA or Swiss citizen outside of both the EEA and Switzerland can prove his/her nationality by any other means (e.g. by presenting an expired passport or national identity card, or a citizenship certificate), he/she must be permitted to enter the EEA or Switzerland. An EEA or Swiss citizen who is unable to demonstrate his/her nationality satisfactorily must nonetheless be given 'every reasonable opportunity' to obtain the necessary documents or to have them delivered within a reasonable period of time.

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Wikipedia