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Baby & child.

Updated: 2 Sep 2021

Booster seats - are you breaking the law?

Confused about the law on booster seats? Find out when you can use them, plus all you need to know about high-back booster seats – including our top five.
Hannah Fox

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.

It's legal to use booster seats or booster cushions as car seats for children, but the rules differ depending on whether they're old or new. 

Some booster cushions are approved for use with children weighing 15kg or more, but any new ones on the market can only be used with children weighing more than 22kg, and taller than 125cm.

Find out more about car seat law in the UK

When can booster seats still be used?

From 1 March 2017 backless booster seats can now only be used with 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 15 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 15 and 36kg, so you’ll need to check the label before you buy.

What is a booster seat?

Booster seats are backless cushions that you can use as a child car seat.

A booster seat can help to raise a child's body to a height suitable for using the adult seatbelt. Some have ‘horns’ that help the car’s adult seatbelt 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 seat's protective 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.

child in booster seat

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-back booster seat instead, whenever possible.

The booster seats we've tested have not proved as safe in our crash tests as high-back 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

In February 2021, we surveyed 2,021 parents with children under five. A worrying number of parents (37%) believe that a backless booster seat offers the same protection as a high-back booster – 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.

Some manufacturers of backless boosters argue that many modern cars have side airbags or other safety features, which should help to protect the occupants. 

backless booster seat in car

Our child car seat experts disagree, as 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 seatsfind 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 to keep older children in high-back car seats for longer, and may encourage parents to swap their backless booster cushion for a high-back booster seat instead.

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

Booster seats in taxis

London taxi cab

If you’re travelling with your child and need to use a backless booster cushion – for example in a taxi, or if 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 weighing less than 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. Many consider this to be the safest position in a vehicle.
  • Some experts consider a backless booster cushion with 'horns' a better choice, as the horns help hold the adult seatbelt in place across your child’s tummy.
  • Remember, it's illegal not to use a child car seat – not to mention unsafe.

MiFold Grab and Go review – is it safe? Read our review and find out before you splash out.

High-back booster seats

high back booster seat

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-back booster seat.

These child car seats are approved for use with children from 15-36kg, which is around three and a half to 12 years of age (around 36kg, or 135cm tall).  

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 your child's 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 they're kept secured in the seat by a five-point harness, while a Group 2/3 seat uses the car's adult seatbelt to keep a child in place. Some may also be using an extended rear-facing seat, which will keep your child rearward facing until they're 105cm tall.

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

There are some seats available that let you extend the harness, or have other innovative ways of dealing with this issue.

Isofix and booster seats

installing isofix base

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 seatbelt in a Group 2/3 seat that helps to distribute 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 that attach to the anchor points in your car.

Find out more in our Isofix car seats guide.

Video: how to fit a high-back booster seat

Best high-backed booster seats

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

Only logged-in Which? members can see the results. If you're not a Which? member, you can get instant access by joining Which? today.

Top five high-backed booster seats

  • 75%

    This Group 2/3 high-back booster seat has advanced side-impact protection and an additional attachment point to help ensure the best 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.

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  • 74%

    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, 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.

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  • 74%

    Billed as being ideal for adapting to your growing child, this i-Size high-backed booster car seat should safely take your child from four to 12 years old. Read our review to find out how much protection this car seat offers in a crash, and whether it’s worth investing in.

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  • 74%

    This i-Size high-back booster seat is approved to the latest regulations, and is suitable for use with children between 100 and 150cm tall, which is around age three to 12 years. Read the full review to find out how it performed in our tough crash tests.

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  • 71%

    This high-back booster seat isn't far off being a Best Buy, and it’s worth considering for children weighing between 15 and 36kg (roughly three and a half to 12 years old). It gets a good overall safety rating and it’s easy to install using the Isofix mounting points or adult seat belt.

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(Table last updated September 2021)

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 in. 

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, which carries out crash testing on cars to show how well they 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 seatbelt), we crash-test it in each format. 

Find out more about Which? car seat tests.