Originally Posted by
vaillancourt
Do you need a passport if you are travelling between two countries that you are citizen of? For example, could a US citizen travel to the UK without passport if he is also a UK citizen? Technically none of the two countries could deny that person entry...
In addition to the practical matters regarding the airline accepting you for travel (e.g. how would they know you're a UK citizen and thus entitled to entry upon arrival if you do not present a UK passport), you also have the matters of a) US law requires all departing air passengers to have passports, and b) US law requires US citizens to have their US passports on their person when they depart, even if it's not the one they use to check in with.