Six million plane passengers have been denied boarding – even with a valid ticket and passport – as airlines overbook flights. Airlines sometimes sell more tickets for a flight than the number of seats on the aircraft, as they assume that some passengers won鈥檛 show up for it. By overbooking the flight, they avoid leaving empty seats on the plane – but this practice can cause a real problem if everyone turns up. Rhys Jones, travel insurance expert at Go.Compare, said: 鈥淔light overbooking happens more than we鈥檇 like to think, and it can make for an extremely stressful start to any trip for the passengers affected.鈥 Mr Jones said: 鈥淩emember to keep all relevant documents like boarding passes, communications from your airline and receipts for accommodation or meal costs.鈥 READ MORE State pension age could change for millions born in the 1970s He continued: 鈥 Know that you can complain to your airline if necessary, and even, if it isn鈥檛 being resolved, take your complaint to the UK Civil Aviation Authority (CAA) or an Alternative Dispute Resolution (ADR) scheme.鈥 If impacted, check why you were denied boarding. Confirm with the airline that this was because of overbooking, and if possible, ask for written confirmation of this, GoCompare has said. And remember to discuss next steps or compensation. Your airline is responsible for finding another flight for you, as well as offering compensation. This could include a cash sum (the amount depends on the length of your flight and the timings of the alternative flight you are offered), meal vouchers, hotel accommodation, transport costs, or a full refund if you decide not to travel. Complain if necessary, too. If you do want to complain, you should first go directly to the airline, as each carrier will have its own process to deal with your complaint. If they won鈥檛 resolve your complaint, consider contacting the UK Civil Aviation Authority (CAA) or an Alternative Dispute Resolution (ADR) scheme.