Is a passport number required to book an international flight?

If you are denied entry on arrival to a country, the airline that brought you there is held responsible for getting you back to your home country (although they will always try very hard to get you to pay for that return flight).  Typically, then, the airline will require not only a passport but a visa, if required for the destination, before allowing you to board.  But they usually won't demand it at the time you book as they want to close the sale.  says in an answer here that he has flown internationally multiple times in the last year without needing a passport.  As the USA now pretty much requires a passport to enter the country by air, I presume he is not flying to the USA, but if he is, it would be very interesting to know more details.

I fly mostly with Cathay amd their online booking process requires at some point that you enter your passport number. I happen to have two passports, and I usually renew one of them every 6 to 9 months. I usually enter whatever passport number I remember (and lately Chrome and Safari remember for me), and use whichever passport I have handy when traveling. Been doing this for 8 years and never had a problem.

No it is not, I have flown internationally multiple times in the last year and it was never required. I think the only thing you need to worry about is maybe if it concerns a visa.

Sometimes you may need, sometimes you may not.But in whole it doesn’t affect anything, so don’t worry whether you provide old or renewed passport number, at the same time make sure you carry both of your passports while boarding.Thank you.