First, try to speak to someone at the airport or airline immediately. They might be able to sort out your problem straight away.
If they can't help you or you're still unhappy, try to find out who is responsible for what went wrong, as it might not be the airline's fault. Then find out whom you need to complain to.
Note down the names of people you speak to, as well as the dates, times and any other relevant information.
If talking to staff doesn't resolve your issue, the next step is to write to the customer relations department of the airline, the tour organiser or operator of your package holiday, or the airport (depending on whose services you're complaining about).
Check the airline's website to find out the best way of complaining; some airlines prefer you to write, some to email and others for you to fill out an online form.
The Civil Aviation Authority provides tips on the information the letter should contain.
Remember to keep copies of all correspondence.
If you have problems with an airline, whom you complain to will depend on whether you travelled by a charter flight or a scheduled one (see 'Charter and scheduled flights' under Flight services in the UK for more details). With charter flights, the contract is with the tour operator, while with scheduled flights, it's with the airline itself. Note, however, that as well as any contractual claim, you may have claims against the organiser of the holiday under the Package Travel and Linked Travel Arrangements Regulations 2018.
If your flight to or from a UK or EU airport is delayed or cancelled, or if you're not allowed to board (e.g. because the flight is overbooked), you might be protected by rules that entitle you to set levels of compensation. See Your rights: flight cancellations, Your rights: flight delays and Your rights: denied flight boarding.
If you've already written to the airline or airport, and you're not happy with the outcome, you can ask the Civil Aviation Authority to help.
If you've complained to an agent or operator, and you're not happy with the response, find out if the company is a member of a trade association. These have arbitration procedures for customer complaints.
The 2 main trade associations are the Association of Independent Tour Operators (AITO) and the Association of British Travel Agents (ABTA).
Both can offer an independent dispute settlement service in which a third party tries to mediate quickly in difficult disputes.
For more information on consumer and trade organisations, see Consumer bodies for holiday complaints.