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If I have three expired passports (UK, Italian, USA), how do I "prove" my nationality? I don't need passports to travel at this point in my life, I have a disability. But I do wonder how, if need be, that I can prove my citizenship of any of these countries if all I have is expired passports? Thank you.

An expired, as opposed to cancelled, passport is usually accepted as evidence of the corresponding nationality. (There are exceptions, such as a passport issued to a foreigner.)

Note that a British passport does not prove one is British. It can be contended that it was issued in error, or simply without proper checking.

How did you obtain each of those citizenships?

If you are a naturalised citizen, the certificate of naturalisation proves your nationality.

For the UK, there is an optional confirmation of nationality status document which you can apply for: https://www.gov.uk/government/publicati ... us-form-ns

This is accepted by the passport office as proof of British nationality