In my experience you don't need a passport to book a flight but you do for on-line check in. With Easyjet that is from 3 months before up until 40 mins before departure.t does vary from airline to airline but I'm pretty sure you can book the flight without the passport number but you will need to fill in details online (via your flight booking) including passport number before your flight. It may be required a certain number of days prior to the flight, so do check that. Kay.