Can I travel with a passport from a different country of nationality than the one declared when buying the plane tickets?

I'm a dual citizen, and I've had enough headaches with the whole situation... but when it comes to booking, I've never had any issues. However, I've been told by airport staff that once you check in, you can't simply switch passports - you might be able to do it if you go ask a member of the airport staff to change it, but I've never needed to try it. Another important thing is that when you're at customs you only show one passport - if you show both, it can actually result in a passport getting confiscated (again, these are the words of airport staff). And my apologies if this is obvious, but if you are travelling into a country where you are a citizen, you need to show the passport from that country, provided you have a passport from that country - if not, you need to have some other way of showing that you are a citizen of that country.

Summary:

Yes. You'll need to show your new passport to the airline. They will not care that you had already told them about a different passport. You may be able to update the passport associated with your booking online, before you check in.

Discussion:

I think the assumption that it is costly to change passport details on your record is false. When you check in, just say, "I have a new passport, here it is." In fact, you can just say "here's my passport"; Nobody cares that it is new. I do this all the time, checking in with a passport other than the one the airline already knows about; nobody even blinks.

Some "nickel-and-dime" (that is, budget) airlines charge, apparently, if the clerk has to enter passport details during check in. Or, more precisely, if the clerk has to swipe your passport through the scanner. To avoid this charge, you should visit the company's web site after you get your new passport, and look up your reservation. You ought to be able to update your passport information online, avoiding the charge.

Anyway, if you are using the new nationality to travel somewhere without a visa, you will have to show that passport to the airline or they won't let you on the plane. You need to prove to them that you have the documents you need to enter the country, because if you don't have those documents, the airline will be fined heavily for bringing you there. Therefore, if you do not show the new passport, you'll need to show a visa, which you do not have.

There are plenty of posts here and elsewhere on the internet explaining how this works. The basic rules are this:

1. When you check in, show the documents that you will use to enter your destination country. In the case of a transit, you might need to show more than one passport, if, for example, one requires a visa at the transit airport and the other requires a visa at the destination.

2. When you enter any country, show the documents that give you the best advantage.

3. When you leave any country, if there is passport control on exit, show the documents that you used to enter. One exception: if you are a national of the country you are leaving, you should usually show those documents, even if you entered with different documents.

There may be some exceptions to these rules, especially if one of your nationalities is of a country with restrictions against dual nationality. I have traveled around the EU and between the US and Canada filtering these principles, without any trouble whatsoever.

You will likely be asked to show your passport when checking in for the flight. Flights to countries that require advance travel notice (e.g., UK, USA, Canada) will require a manual check of documents by airline staff, even if you entered the information online.

Just present the new passport when at the airport, and that's the information that will be passed to the destination immigration authorities.

I once had a similar issue: the passport was reissued and thus the passport data has changed. Moreover, I had two flights from two different airlines booked separately (with a day stopover). Not finding a possibility to change the booking data online, I just emailed both airlines via an email from their website (or via some contact form, I do not remember exactly), and both replied that they have changed the passport details in my booking. No fee was charged. Later, on registration, I was prepared to show both passports and the email thread printed, but no one asked anything.

I am no travel expert. But I wish to present a hypothetical to you because everybody else here seems to think you can just forget about it for the most part.

A few years ago, I was fined £10 by a low-budget airline in the UK for failing to pre-enter my passport details into my booking online, because now they had to do it themselves at the "check-in" desk (which was by this point less of a "check-in" desk and more of a "see whether you've completed online check-in yourself and if not take more money from you" desk).

I can't imagine that this wouldn't apply if you'd put in what essentially turned out to be the wrong details. So, yes, sometimes airlines do care. Still, £10 is not a lot and, in those cases, you can probably update the passport details online before check-in anyway.

Ultimately, it's going to depend on where you are and who you're flying with. It's a shame you didn't name the airline.