If you are looking for a new car there are plenty of things to consider.

Size, running and insurance costs, mileage and price are all top of the agenda - but a small bit of industry code could help you to avoid making a potentially dangerous and expensive mistake.

All used cars for sale come with an insurance category rating that can tell used car buyers if the car has been in an accident or needs to be repaired before it is fit to drive.

Used car site Auto Trader says if your car is listed as a Category A or Category B then it means it is not legally allowed on the road and could potentially be dangerous to those who drive them.

The site says: "Category A and Category B cars are so badly damaged, Auto Trader will not allow them to be advertised for sale, and strongly advise you not to buy one."

Category A cars are so badly damaged they should be crushed and never re-appear on the road, while Category B cars mean there is extensive damage and only undamaged reclaimed parts should be allowed to be used in other road-going vehicles.

However, while both categories are banned from Auto Trader listings buyers should also be aware of two other categories of car on the site.

Category S and Category N cars (formerly C and D respectively) could leave buyers with big repair bills.

Category S cars are those that have suffered structural damage in a crash and will need to be repaired before you can drive it. The cost of the repair could be more than the price of the car, meaning it it officially an insurance write-off.

A Category N car is also a damaged car and while that may be cosmetic, it could also mean damage to breaks or steering, which could render the vehicle undriveable.

Not only could you face big repair bills, your insurance is also likely to be higher.

Auto Trader says: "Before you buy it, ask to see documentary evidence of what happened to the car and what repairs it has had. Consider having a third-party inspection done to give it the all-clear. Also, check how much it will cost to insure, as some insurance companies will charge more for Cat S/N cars, and others won’t cover them at all."

To ensure you are not caught out when buying a new car all buyers should get a HPI check, which can tell you if the car has been a write-off, if there is outstanding finance on the vehicle, and the MOT history of the car.

You can get a HPI check here.