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Booster seats - are you breaking the law?

By Lisa Galliers

Confused about the law on booster seats? Booster seat rules changed in 2017, but these seats are still perfectly legal if used correctly.

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UK law states that children must use a child car seat until they're 12 years old or 135cm/4ft 5in tall, whichever comes first.

Booster seats, or booster cushions, can be legally used as car seats for children, but the rules differ depending on whether products are old or news. Some booster cushions are approved for use with children weighing 15kg or more, but any new products on the market can only be used with children who weigh 22kg and are 125cm in height.

Find out more about car seat regulations - car seat law in the UK

Read on to find out when booster seats can be used and why we don’t recommend them – plus all you need to know about high-backed booster seats including our top five, and why Isofix works differently with Group 2/3 seats for older children.

When can booster seats still be used? 

From 1 March 2017 the rules on backless booster seats changed. These seats can now only be made for children taller than 125cm and weighing more than 22kg.

However, backless booster seats made and bought before this date are still legal and approved for use by children weighing between 15kg and 36kg. So, if you own a backless booster seat bought before this date you can still use it for a child within this weight range.

Shops can also sell backless booster seats that are approved for use by children weighing between 15kg and 36kg, so you’ll need to check the label when you buy a seat.

And don’t believe what you read elsewhere. Which? child car seat expert Lisa Galliers says: ‘Since we first reported on the expected rules change for booster seats, there have been a lot of confusing reports, especially on social media, about the changes.

‘Some we’ve spotted have been incorrect or inaccurate. We’ve seen posts saying booster seats will be illegal for a certain age or weight, or implying there are no new regulations regarding booster seats, which is confusing for consumers.

‘To be clear, the new rules only apply to any new products entering the market.’

‘Since our story back in April 2016 when we first reported the upcoming changes, we’ve seen a lot more people talking about child car seats and it’s really opened up the discussion about the safest way to transport children, which is extremely positive.’

What is a booster seat? 

Booster seats are backless cushions that can be used as a child car seat.

A booster seat can help to raise a child's body to a height suitable for use with the adult seatbelt. Some have ‘horns’ that help the car’s adult seat belt to sit across your child’s tummy.

You can buy booster seats that are simply the cushion part. But some Group 2/3 car seats also convert to a booster cushion for Group 3 mode. Essentially, this removes the protective car seat sides (Group 2) and turns the seat into a booster cushion.

Booster seats are temptingly cheap – some start from as little as £6 – and there’s no doubt they’re convenient. But while using any seat is better than using none, booster seats offer very little protection in a crash, particularly if you’re hit from the side.

Should a child be in a booster seat? 

Our car seat experts and many others believe these backless cushions are not the best way to transport children, and recommend using a high-backed booster seat instead, whenever possible.

The booster seats we've tested have not proved as safe in our crash tests as high-backed booster seats with a full-length back and 'wings', as these provide extra protection for the head and chest in a side-impact crash.

Find those that came top in our crash tests in our car seat reviews

35%Parents who incorrectly think a booster cushion offers the same protection as a high-backed booster seat

A worrying number of parents (35%*) believe that a backless booster seat offers the same protection as a high-backed booster seat – but this isn't the case.

Booster cushions raise children up, but some struggle with keeping the shoulder strap of a three-point adult seatbelt in the right place on a child and provide virtually no side-impact protection if you’re involved in a side-on collision.

There is an argument by some backless booster manufacturers that many modern cars have side airbags or other safety features which should help protect the occupants. Our child car seat experts disagree. Not every car has added safety features. There’s also a risk that a young child sitting on a booster cushion could easily wriggle out of position – we’ve seen this happen in our fitting tests. What’s more, a young child may not be tall enough for their head to reach the side airbags.

We’ve tested a handful of multi-group seats that convert to a backless booster seat in Group 3 mode and these are Don't Buys because of their poor crash test results, especially for side-impact crashes. A good result in any other part of our crash tests cannot compensate for this.

Some booster cushions have performed so badly in our crash tests, the results meant the score was instantly downgraded to a 0% Don't Buy – our lowest possible score.

Don't Buy car seats - find out which car seats to avoid

Booster seats for older children 

A booster seat is better than no seat. But the change to booster seat rules should help keep older children in high-backed car seats for longer and may encourage parents to swap their backless booster cushion for a high-backed booster seat instead.

However, we can see that a backless booster seat is an appealing option for parents who may have to try to squeeze a third child into the middle seat in the back of a car, occasionally, or for a short journey in a taxi.

Booster seats in taxis 

If you’re travelling with your child and do need to use a backless booster cushion - maybe you’re heading off on holiday and want one to use in a taxi to the airport, or maybe you regularly give a child a lift to school - then read on for tips below on how to use one:-

  • Avoid using a backless booster cushion with young children if at all possible, especially those as young as 15kg (around three years old). While some seats are approved for use with such light children, our experts don’t think they offer the best protection.
  • Use the booster cushion in the middle back seat of a vehicle if you can. This is considered by many as the safest position in a vehicle. You could sit either side of your child.
  • A backless booster cushion with 'horns', to help hold the adult seat belt in place across your child’s tummy, is considered a better choice by some experts.
  • Remember, it's illegal not to use a child car seat not to mention unsafe.

Mifold – is it safe? Read our review and find out before you splash out.

High-backed booster-seats 

When your toddler reaches 18kg (usually around four years old), or is taller than 105cm, you'll need to swap to the next stage of child car seat - a Group 2/3 seat. This is what’s known as a high-backed booster seat.

These child car seats are approved for use for children from 15kg-36kg, which is around three-and-a-half-years-old all the way up to 12 years of age (around 36kg, or 135cm in height).  

There's no need to swap to a Group 2/3 seat until your child has outgrown his or her toddler/Group 1 seat. This will happen when your child reaches around 18kg, or is too tall for the seat (i-Size-approved seats usually go up to 105cm). You'll be able to see this when his or her eyes are level with the top of the seat.  

Some parents prefer to keep their child in a toddler seat for as long as possible because he or she is kept secured in the seat by a five-point harness, rather than swapping to a Group 2/3 seat, which uses the car's adult seat belt to keep a child in place.  Some may also be using an extended rear-facing seat, which will keep your child rear-facing until 105cm.

Our car seat experts recommend using the lowest group seat for as long as possible.  

There are some seats available which have extended harness use, plus other seats which also have innovative ways of dealing with this issue.

Isofix and booster seats

Isofix connectors on a high-backed booster seat don’t work in the same way as they do for baby or toddler car seats. The connectors are used mainly to keep the seat stable and securely in place when it’s not in use, so it doesn’t fly forward and hit you if you have to brake suddenly or are involved in a crash and the kids aren't on board.  

It’s the adult seat belt in a Group 2/3 seat that helps distributes the force of a crash away from a child’s body.  Some seats in this category have Isofit connectors, rather than Isofix, They're similar but are more like soft latches, or hooks which attach to the anchor points in your car.

Best high-backed booster seats 

Below, you can see our pick of five high-scoring and recently tested high-backed booster seats.   

Remember, not all car seats fit in all cars, so make sure the car seat you choose fits before you buy it.  

Only logged-in Which? members can see the results and read full reviews of each model. If you're not a Which? member, you can get instant access by joining Which? today.

Top five high-backed booster seats


You'll have no issues installing this lightweight high-backed booster seat, which is approved for use from 15kg to 36kg (approximately four to 12 years old). It’s easy to fit, and you can install it using the adult seat belt and Isofix, which means it stays secure in the car even when not in use. It has a nice sitting position and a large, well-padded seat, plus it doesn’t take up much room in the back of your car.


This spacious Group 2/3 high-backed booster seat is approved for use with children from 15kg to 36kg (around three or four years old to 12 years of age). It offers good frontal-impact protection and excellent side-crash protection due to the Isofit connectors that are designed with a sideways movement to help push the car seat away from the force of a side-impact crash.


This Group 2/3 high-back booster seat has advanced side impact protection and the a fourth-attachment point to help ensure the optimal lap belt position. It gets a good four-star rating overall for safety and if you use the optional Isofix connectors it means you don't need to remember to secure the car seat when it's not in use.


This car seat has no major weaknesses and is among the cheapest Best Buy Group 2/3 car seats currently available. The level of overall crash protection is good, and it's straightforward to install and easy to use. You can adapt it to comfortably fit your growing child, and the Isofix connectors allow you to securely attach it to your car, even when it's not in use.

How Which? tests child car seats 

In our unique tests, each car seat endures a front crash, equivalent to a head-on collision at around 40mph, and a side crash equivalent to a car crashing into another one at 30mph. We repeat the crashes again and again, with the seat fixed in the different modes it can be used. 

We can go through as many as 20 samples of the same seat to get the final score. 

Crash tests Our experts have specially designed the crash tests, making them more demanding than the legal minimum standard requires. They’re derived from Euro NCAP – it carries out crash testing on cars to show how well cars protect occupants in severe accidents. We do a similar thing for car seats and feel this more accurately reflects what could happen in a real crash. 

Crash test dummies These are wired up to record the crash forces on the most vulnerable parts of the body and accurately indicate the risk of injury a real child could have in a crash. 

Multi-group seats If a car seat can be used in a number of different ways and attached by different methods (Isofix or car’s seat belt), we crash test it in each format. 

Find out more about how Which? tests child car seats.


* In March 2020, we surveyed 1,800 parents with children under 12.


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