If you've spent a lot of money booking a flight for a holiday or a work trip, you expect a guaranteed seat on that flight, right?

Wrong, unfortunately. 

In fact, in the United Airlines situation the law was broken, but by the passenger who refused to leave the flight, not the airline.

Here’s everything you need to know about overbooked flights and your rights.

Can an airline simply refuse to let me fly?

Yes. When you are on a plane the captain is in charge and their say is final. If they decide they don’t want you on their plane you have to disembark.

There are numerous reasons this could happen from you being deemed drunk and disruptive to a last minute change in the size of the plane meaning there isn’t enough room for everyone.

But, one of the most common reasons is that the airline simply overbooked the flight.

Whatever the reason if you’re asked to get off a flight you can politely explain why you really need to be on that flight – good reasons include weddings, funerals or if you have an important job that you are needed for such as surgery or a court case – but if your appeal is refused you need to get off that flight.

If you refuse to disembark the captain is entitled to seek assistance from airport security and have you forcibly removed from the plane.

The issue in the case of United Airlines is that far too much force was used and there are questions about how they selected that particular passenger to be removed. After all, he was a doctor arguing that he needed to be back at work treating patients the next day.

Is overbooking legal?

Astonishingly, yes. It may be immoral for airlines to sell more tickets than there are seats on a plane but it is completely legal.

Airlines argue that overbooking benefits everyone as on any given flight an average of 5% of passengers don’t turn up.

By selling more tickets than seats the airline makes more money, passengers get to book on to a flight that might technically be full and planes are more likely to fly full, allowing airlines to keep fares low.

What usually happens when a flight is overbooked?

Typically, an airline will know a flight is overbooked – and too many people have turned up for the flight – before they start boarding.

This means they can start offering people compensation if they agree to take a later flight at the gate, rather than having to try and convince passengers who have already taken their seat to get off the plane.

You’ll be offered compensation and that usually entices enough passengers to get off the plane. But, if no-one volunteers to take a later flight airline staff then start choosing passengers who will not be allowed on the flight.

How do airlines decide who to bump from a flight?

Usually when deciding who to bump from a flight a variety of factors are considered.

These include how much you paid for your ticket, whether you are a member of the airline’s frequent flyer scheme, and what time you checked in.

Your profession may also be considered; if your job means you are going to be urgently needed and a delay would cause major problems.

[Read more: Best credit cards to use on holidays abroad]

Am I entitled to compensation?

If you’re asked, or commanded, to take a later flight you’re entitled by law to compensation.

How much depends on where you’re flying from, where your airline is based, and how long you’re delayed by.

Take up the offer and volunteer to be bumped from a flight and you can negotiate your compensation with the airline staff.

They are usually quite generous as you have helped them avoid having to anger passengers by forcing them not to take the flight. An upgrade to business class plus compensation is fairly normal.

Whether you volunteer or are forced not to take a flight due to overbooking there are laws on the minimum compensation you are entitled to.

EU Flights

If you are flying within Europe or are on an EU-based airline anywhere in the world then you will be awarded:

Length of flight and delay

Minimum compensation

Short-haul flight (less than 1,500km )and a delay of under two hours

€125

Short-haul flight (less than 1,500km) and a delay of more than two hours

€250

Medium haul (1,500km – 3,500km or within the EU over 1,500km) and a delay of under three hours

€200

Medium haul (1,500km – 3,500km or within the EU over 1,500km) and a delay of more than three hours

€400

Long-haul flights (over 3,500km) and a delay of under four hours

€300

Long-haul flights (over 3,500km) and a delay of more than four hours

€600

As well as compensation, your airline must also give you the choice of either an alternative flight – this can be the next available flight or on a date/time that suits you – or a refund.

The amount you are refunded will depend on your ticket and whereabouts in your journey you are. For example, if you are bumped from the outbound leg of a return flight you can get a full refund, but if you are on the return leg you would only get a refund for that part of the journey.

USA

USA rules work on a sliding scale similar to the EU rules.

Length of flight and delay

Minimum compensation

Arrive within one hour of your original arrival time

$0

A domestic flight and your delay is between one and two hours

Double the one-way fare up to a maximum of $675

An international flight with a delay of between one and four hours

Double the one-way fare up to a maximum of $675

A domestic flight and your delay is over two hours

Four times the one-way fare up to a maximum of $1,350

An international flight with a delay of more than four hours

Four times the one-way fare up to a maximum of $1,350

Rest of the world

The rules vary by region and in many parts of the world there are no laws, so it’s down to the airlines to decide.